I found it extremely weird.
When I want to add 2 references together, the second reference becomes below: which I do not want to have
Author 1, Author 2 et al. (year)
Reference Two is Hermann, N ̈olker et al. (2002)
I want:
Author 1 et al. (year)
Reference Two is Hermann et al. (2002)
Below is the printscreen:
Both references together:

When only 1 reference:
I am using:
maxcitenames=2
My full MWE (with 2 references):
\documentclass[12pt,oneside]{book} % Remove draft option to show figures (for final draft), otherwise keep for faster production
\usepackage{enumitem} % macros to modify appearance of 'itemize' environments
\usepackage[english]{babel}
\RequirePackage{xcolor}
\definecolor{darkblue}{rgb}{0, 0, 0.5}
\usepackage[colorlinks=true, allcolors=darkblue]{hyperref}
\usepackage[style=british]{csquotes}
\usepackage[backend=biber,
% style=authoryear,
style=authoryear-comp,
% citestyle=authoryear,
dashed=false,
maxcitenames=2,
maxbibnames=99,
giveninits,
alldates=comp, %for online citing
dateabbrev=false, %for online citing for data format month in full
uniquename=init]{biblatex}
\DeclareNameAlias{sortname}{family-given} %
\usepackage{xpatch}
\renewcommand{\labelnamepunct}{\space} %to delete . after (1997)
\DeclareFieldFormat[article, incollection, unpublished]{title}{#1}
% for title italic
% \DeclareFieldFormat*{title}{\mkbibemph{#1}} %for title italic in all format (conference, paper..)
\DeclareFieldFormat*{title}{#1} %for conference title delete ".."
\DeclareFieldFormat[incollection]{title}{\mkbibemph{#1}} %for incollecion title italic ONLY
\DeclareFieldFormat[thesis]{title}{\mkbibemph{#1}} %for phdthesis title italic ONLY
\DeclareFieldFormat[book]{title}{\mkbibemph{#1}} %for book title italic ONLY
% for title italic
% to delete "In:" for article
\renewbibmacro{in:}{%
\ifentrytype{article}{}{\printtext{\bibstring{in}\intitlepunct}}}
% to delete "In:" for article
% for delete comma after Author A and Aurhor b
\DefineBibliographyExtras{english}{%
\let\finalandcomma\empty
\let\finalandsemicolon\empty
}
\renewcommand{\compcitedelim}{\addsemicolon\space}
\DeclareNameAlias{editorin}{given-family}
\newbibmacro*{byeditor:in}{%
\ifnameundef{editor}
{}
{\printnames[editorin]{editor}%
\setunit{\addcomma\space}%
\usebibmacro{editorstrg}%
\clearname{editor}}}
\xpatchbibdriver{inbook}
{\usebibmacro{in:}%
\usebibmacro{bybookauthor}%
\newunit\newblock
\usebibmacro{maintitle+booktitle}%
\newunit\newblock
\usebibmacro{byeditor+others}}
{\usebibmacro{in:}%
\usebibmacro{bybookauthor}%
\newunit\newblock
\usebibmacro{byeditor:in}%
\newunit\newblock
\usebibmacro{maintitle+booktitle}%
\newunit\newblock
\usebibmacro{byeditor+others}}
{}{}
\xpatchbibdriver{incollection}
{\usebibmacro{in:}%
\usebibmacro{maintitle+booktitle}%
\newunit\newblock
\usebibmacro{byeditor+others}}
{\usebibmacro{in:}%
\usebibmacro{byeditor:in}%
\setunit{\labelnamepunct}\newblock
\usebibmacro{maintitle+booktitle}%
\newunit\newblock
\usebibmacro{byeditor}}
{}{}
\xpatchbibdriver{inproceedings}
{\usebibmacro{in:}%
\usebibmacro{maintitle+booktitle}%
\newunit\newblock
\usebibmacro{event+venue+date}%
\newunit\newblock
\usebibmacro{byeditor+others}}
{\usebibmacro{in:}%
\usebibmacro{byeditor:in}%
\setunit{\labelnamepunct}\newblock
\usebibmacro{maintitle+booktitle}%
\newunit\newblock
\usebibmacro{event+venue+date}%
\newunit\newblock
\usebibmacro{byeditor+others}}
{}{}
\xpatchbibdriver{phdthesis}
{\usebibmacro{in:}%
\usebibmacro{maintitle+booktitle}%
\newunit\newblock
\usebibmacro{event+venue+date}%
\newunit\newblock
\usebibmacro{byeditor+others}}
{\usebibmacro{in:}%
\usebibmacro{byeditor:in}%
\setunit{\labelnamepunct}\newblock
\usebibmacro{maintitle+booktitle}%
\newunit\newblock
\usebibmacro{event+venue+date}%
\newunit\newblock
\usebibmacro{byeditor+others}}
{}{}
\DeclareDelimFormat[textcite]{postnotedelim}{\addsemicolon\space}
\DeclareDelimFormat[parencite]{postnotedelim}{\addcolon\space}
\DeclareFieldFormat{postnote}{\printtext[bibhyperref]{#1}}
\DeclareFieldFormat{multipostnote}{#1}
\DeclareFieldFormat{url}{Available at\addcolon\space\url{#1}}
\DeclareFieldFormat{urldate}{\mkbibbrackets{\bibstring{urlseen}\space#1}}
\DeclareFieldFormat[online]{title}{\mkbibemph{#1}}
\DefineBibliographyStrings{english}{%
urlseen = {Accessed},
}
% below is to keep 'english' format as babel
\DefineBibliographyExtras{english}{%
\protected\def\mkdaterangecomp{%
\mkdaterangetrunc{long}}%
\protected\def\mkdaterangeterse{%
\mkdaterangetrunc{short}}%
\protected\def\mkdaterangecompextra{%
\mkdaterangetruncextra{long}}%
\protected\def\mkdaterangeterseextra{%
\mkdaterangetruncextra{short}}%
\protected\def\mkbibdatelong#1#2#3{%
\iffieldundef{#3}
{}
{\thefield{#3}%
\iffieldundef{#2}{}{\nobreakspace}}%
\iffieldundef{#2}
{}
{\mkbibmonth{\thefield{#2}}%
\iffieldundef{#1}{}{\space}}%
\iffieldbibstring{#1}
{\bibstring{\thefield{#1}}}
{\dateeraprintpre{#1}\stripzeros{\thefield{#1}}}}%
}
\usepackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@article{hermann2005guest,
title={Guest editors' introduction: An introduction to interactive sonification},
author={Hermann, Thomas and Hunt, Andy},
journal={IEEE multimedia},
year={2005},
}
@inproceedings{hermann2001hand,
title={Hand postures for sonification control},
author={Hermann, Thomas and N{\"o}lker, Claudia and Ritter, Helge},
booktitle={International Gesture Workshop},
year={2002},
}
\end{filecontents*}
\addbibresource{\jobname.bib}
\begin{document}
Reference One is \textcite{hermann2005guest}.
Reference Two is \textcite{hermann2001hand}
\printbibliography
\appendix
\include{chapters/99-appendix}
\end{document}
My Full MWE (with 1 reference):
\documentclass[12pt,oneside]{book} % Remove draft option to show figures (for final draft), otherwise keep for faster production
\usepackage{enumitem} % macros to modify appearance of 'itemize' environments
\usepackage[english]{babel}
\RequirePackage{xcolor}
\definecolor{darkblue}{rgb}{0, 0, 0.5}
\usepackage[colorlinks=true, allcolors=darkblue]{hyperref}
\usepackage[style=british]{csquotes}
\usepackage[backend=biber,
% style=authoryear,
style=authoryear-comp,
% citestyle=authoryear,
dashed=false,
maxcitenames=2,
maxbibnames=99,
giveninits,
alldates=comp, %for online citing
dateabbrev=false, %for online citing for data format month in full
uniquename=init]{biblatex}
\DeclareNameAlias{sortname}{family-given} %
\usepackage{xpatch}
\renewcommand{\labelnamepunct}{\space} %to delete . after (1997)
\DeclareFieldFormat[article, incollection, unpublished]{title}{#1}
% for title italic
% \DeclareFieldFormat*{title}{\mkbibemph{#1}} %for title italic in all format (conference, paper..)
\DeclareFieldFormat*{title}{#1} %for conference title delete ".."
\DeclareFieldFormat[incollection]{title}{\mkbibemph{#1}} %for incollecion title italic ONLY
\DeclareFieldFormat[thesis]{title}{\mkbibemph{#1}} %for phdthesis title italic ONLY
\DeclareFieldFormat[book]{title}{\mkbibemph{#1}} %for book title italic ONLY
% for title italic
% to delete "In:" for article
\renewbibmacro{in:}{%
\ifentrytype{article}{}{\printtext{\bibstring{in}\intitlepunct}}}
% to delete "In:" for article
% for delete comma after Author A and Aurhor b
\DefineBibliographyExtras{english}{%
\let\finalandcomma\empty
\let\finalandsemicolon\empty
}
\renewcommand{\compcitedelim}{\addsemicolon\space}
\DeclareNameAlias{editorin}{given-family}
\newbibmacro*{byeditor:in}{%
\ifnameundef{editor}
{}
{\printnames[editorin]{editor}%
\setunit{\addcomma\space}%
\usebibmacro{editorstrg}%
\clearname{editor}}}
\xpatchbibdriver{inbook}
{\usebibmacro{in:}%
\usebibmacro{bybookauthor}%
\newunit\newblock
\usebibmacro{maintitle+booktitle}%
\newunit\newblock
\usebibmacro{byeditor+others}}
{\usebibmacro{in:}%
\usebibmacro{bybookauthor}%
\newunit\newblock
\usebibmacro{byeditor:in}%
\newunit\newblock
\usebibmacro{maintitle+booktitle}%
\newunit\newblock
\usebibmacro{byeditor+others}}
{}{}
\xpatchbibdriver{incollection}
{\usebibmacro{in:}%
\usebibmacro{maintitle+booktitle}%
\newunit\newblock
\usebibmacro{byeditor+others}}
{\usebibmacro{in:}%
\usebibmacro{byeditor:in}%
\setunit{\labelnamepunct}\newblock
\usebibmacro{maintitle+booktitle}%
\newunit\newblock
\usebibmacro{byeditor}}
{}{}
\xpatchbibdriver{inproceedings}
{\usebibmacro{in:}%
\usebibmacro{maintitle+booktitle}%
\newunit\newblock
\usebibmacro{event+venue+date}%
\newunit\newblock
\usebibmacro{byeditor+others}}
{\usebibmacro{in:}%
\usebibmacro{byeditor:in}%
\setunit{\labelnamepunct}\newblock
\usebibmacro{maintitle+booktitle}%
\newunit\newblock
\usebibmacro{event+venue+date}%
\newunit\newblock
\usebibmacro{byeditor+others}}
{}{}
\xpatchbibdriver{phdthesis}
{\usebibmacro{in:}%
\usebibmacro{maintitle+booktitle}%
\newunit\newblock
\usebibmacro{event+venue+date}%
\newunit\newblock
\usebibmacro{byeditor+others}}
{\usebibmacro{in:}%
\usebibmacro{byeditor:in}%
\setunit{\labelnamepunct}\newblock
\usebibmacro{maintitle+booktitle}%
\newunit\newblock
\usebibmacro{event+venue+date}%
\newunit\newblock
\usebibmacro{byeditor+others}}
{}{}
\DeclareDelimFormat[textcite]{postnotedelim}{\addsemicolon\space}
\DeclareDelimFormat[parencite]{postnotedelim}{\addcolon\space}
\DeclareFieldFormat{postnote}{\printtext[bibhyperref]{#1}}
\DeclareFieldFormat{multipostnote}{#1}
\DeclareFieldFormat{url}{Available at\addcolon\space\url{#1}}
\DeclareFieldFormat{urldate}{\mkbibbrackets{\bibstring{urlseen}\space#1}}
\DeclareFieldFormat[online]{title}{\mkbibemph{#1}}
\DefineBibliographyStrings{english}{%
urlseen = {Accessed},
}
% below is to keep 'english' format as babel
\DefineBibliographyExtras{english}{%
\protected\def\mkdaterangecomp{%
\mkdaterangetrunc{long}}%
\protected\def\mkdaterangeterse{%
\mkdaterangetrunc{short}}%
\protected\def\mkdaterangecompextra{%
\mkdaterangetruncextra{long}}%
\protected\def\mkdaterangeterseextra{%
\mkdaterangetruncextra{short}}%
\protected\def\mkbibdatelong#1#2#3{%
\iffieldundef{#3}
{}
{\thefield{#3}%
\iffieldundef{#2}{}{\nobreakspace}}%
\iffieldundef{#2}
{}
{\mkbibmonth{\thefield{#2}}%
\iffieldundef{#1}{}{\space}}%
\iffieldbibstring{#1}
{\bibstring{\thefield{#1}}}
{\dateeraprintpre{#1}\stripzeros{\thefield{#1}}}}%
}
\usepackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@article{hermann2005guest,
title={Guest editors' introduction: An introduction to interactive sonification},
author={Hermann, Thomas and Hunt, Andy},
journal={IEEE multimedia},
year={2005},
}
@inproceedings{hermann2001hand,
title={Hand postures for sonification control},
author={Hermann, Thomas and N{\"o}lker, Claudia and Ritter, Helge},
booktitle={International Gesture Workshop},
year={2002},
}
\end{filecontents*}
\addbibresource{\jobname.bib}
\begin{document}
% Reference One is \textcite{hermann2005guest}.
Reference Two is \textcite{hermann2001hand}
\printbibliography
\appendix
\include{chapters/99-appendix}
\end{document}

Reference Two is Hermann et al. (2002). I wantAuthor1 et al. (year)– aan Oct 16 '19 at 22:58mincitenames=1. – Bernard Oct 16 '19 at 23:01mincitenames=1,and become\usepackage[backend=biber, style=authoryear-comp, dashed=false, maxcitenames=2, maxbibnames=99, mincitenames=1, giveninits, alldates=comp, dateabbrev=false, uniquename=init]{biblatex}. Still the same error. – aan Oct 16 '19 at 23:02biblatextries to disambiguate potentially ambiguous lists of names by adding more names to make them unique. You needuniquelist=falseif you want to turn off that feature. This feature is context-dependent (since the ambiguity depends on whether or not you cited works with similar author lists), so it only comes into play once you cite a second work (with a similar author list). – moewe Oct 17 '19 at 05:34