Set comma separator for multiple references in a Beamer presentation with BibTeX

Question

I'd like to cite two sources on the same line. They're supposed to appear in superscript. There is no separator between them, just a whitespace, which makes it look like '12' instead of the desired '1, 2'. Here's an MWE:

\documentclass{beamer}
\usepackage[style=verbose-ibid,backend=bibtex]{biblatex}

\usepackage{filecontents}
\begin{filecontents}{refs.bib}
@article{ref_a,
    author = {A},
    title = {a},
    year = {1970},
}
@book{ref_b,
    author = {B},
    title = {b},
    year = {1970},
} 
\end{filecontents}

\bibliography{refs}
\begin{document}

\begin{frame}\frametitle{frametitle}
text.

\begin{itemize}
    \item item \autocite{ref_a} \autocite{ref_b}    
\end{itemize}
\end{frame}
\end{document}

So if I'm using \autocite{ref_a} \autocite{ref_b}, I get '12' instead of '1, 2'. And if I'm using \autocite{ref_a, ref_b}, I get '1' and the footnote lists the two references in the same entry, separated by a semicolon. And if I'm using \autocite{ref_a}, \autocite{ref_b}, then this doesn't work either, of course. What I'd like is two separate entries and '1, 2'. Is there a way to set the separator for such references in Beamer with BibTeX?

Answer 1

Since biblatex just typesets its \footcite (which is what \autocite comes down to with your settings) using the normal \footnote command, this question could be reformulated independently of biblatex as: "How do I place a comma between directly consecutive footnote markers?" (This is purely a beamer question in this case and has nothing to do with biblatex.)

I will, however, try to answer a different question, which I feel is the question you wanted to ask. Namely: "How do I get numeric citations with consistent numbering and full footnote references with biblatex in beamer?"

This question has been addressed with devilishly clever tricks by Audrey in Footnotes disappear when using overlays in beamer, so here is just a slight variation of that answer

\documentclass{beamer}
\usepackage[backend=bibtex, style=numeric-comp, citetracker=true]{biblatex}

\makeatletter
\newtoggle{cbx@togcite}
\DeclareCiteCommand{\sfcite}[\cbx@superscript]%
  {\usebibmacro{cite:init}%
   \let\multicitedelim=\supercitedelim
   \iffieldundef{prenote}
     {}
     {\BibliographyWarning{Ignoring prenote argument}}%
   \iffieldundef{postnote}
     {}
     {\BibliographyWarning{Ignoring postnote argument}}}
  {\usebibmacro{citeindex}%
   \ifciteseen
     {\ifnumequal{\value{page}}{\csuse{cbx@page@\thefield{entrykey}}}
       {}
       {\ifnumequal{\value{framenumber}}{\csuse{cbx@frame@\thefield{entrykey}}}
          {\usebibmacro{sfcite}}
          {}}}
     {\usebibmacro{sfcite}}%
   \usebibmacro{cite:comp}}
  {}
  {\usebibmacro{cite:dump}}

\newbibmacro*{sfcite}{%
  \csnumgdef{cbx@page@\thefield{entrykey}}{\value{page}}%
  \csnumgdef{cbx@frame@\thefield{entrykey}}{\value{framenumber}}%
  \xappto\cbx@citehook{%
    \global\toggletrue{cbx@togcite}%
    \noexpand\cbx@beamersfcite@footnote{\thefield{entrykey}}}}

% vary bad hack to get the footnote format as desired
\newcommand\cbx@footnotewithmark[2]{%
  \def\@makefntext##1{%
    \parindent 1em\noindent%
    \raggedright
    \hbox to 1.8em{\mkbibbrackets{#1}\hfil}\bibfootnotewrapper{#2}\par}%
  \footnotetext{we don't need this}}

\DeclareCiteCommand{\cbx@beamersfcite@footnote}
  {}
  {\cbx@footnotewithmark{%
     \usebibmacro{cite:init}%
      \usebibmacro{cite:comp}%
      \usebibmacro{cite:dump}}
    {\usedriver
      {\DeclareNameAlias{sortname}{default}}
      {\thefield{entrytype}}}}
  {}
  {}

\newrobustcmd{\cbx@superscript}[1]{%
  \mkbibsuperscript{\mkbibbrackets{#1}}%
  \cbx@citehook%
  \global\let\cbx@citehook=\@empty}

\let\cbx@citehook=\@empty

\DeclareMultiCiteCommand{\sfcites}[\cbx@superscript]{\sfcite}{\supercitedelim}
\DeclareAutoCiteCommand{sfcite}[f]{\sfcite}{\sfcites}
\makeatother

\ExecuteBibliographyOptions{autocite=sfcite}


\addbibresource{biblatex-examples.bib}
\begin{document}
\begin{frame}
\frametitle{frametitle}
text.

\begin{itemize}
  \item item \autocite{sigfridsson,worman}
  \item ipsum \autocite{sigfridsson}
\end{itemize}
\end{frame}
\end{document}