In the question Biblatex: No \postnotedelim for citations that aren't numerals, I learned how to remove a postnote delimiter when the citation doesn't contain a numeral, by adding this to the preamble:
\renewcommand{\postnotedelim}{\iffieldnums{postnote}{\addcolon}{\addspace}}
However, this also removes the delimiter between a regular citation and a "comment" style citation (such as "NN p.c.") (cf. Add custom information to text citations with biblatex).
The question is, how do I make sure that the delimiter between the "regular" citations and the "comment" is the same character (comma, semicolon) as used to delimit the "regular" citations?
\documentclass{article}
\usepackage[style=authoryear]{biblatex}
\DeclareFieldFormat{postnote}{#1}
\renewcommand{\multicitedelim}{\addcomma\space}
\renewcommand{\postnotedelim}{\iffieldnums{postnote}{\addcolon}{\addspace}}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@book{lennon1972,
AUTHOR = "John Lennon",
TITLE = "Who did what in the Beatles",
YEAR = "1972"}
@book{starkey1994,
AUTHOR = "Richard Starkey",
TITLE = "I'm the drummer",
YEAR = "1994"}
@ONLINE{archiveZ,
TITLE = "Archive Z",
SHORTHAND = "Archive Z",
URL = "http://www.archivez.com",
URLDATE = "2013-03-22"}
\end{filecontents}
\addbibresource{\jobname.bib}
\begin{document}
I know from books and people I have talked to that George Harrison
played the guitar in the Beatles \parencites(Paul McCartney p.c.)
[48]{lennon1972}[103]{starkey1994}[no. 58]{archiveZ}
\end{document}

The citation should ideally look like this:
(Lennon 1972:48, Starkey 1994:103, Archive Z no. 58, Paul McCartney p.c.)

\renewcommand{\postnotedelim}{\iffieldnums{postnote}{\addcolon}{\addspace}}was created to ensure that no delimiting character was used, as addressed in http://tex.stackexchange.com/questions/103770/biblatex-no-postnotedelim-for-citations-that-arent-numerals. I'll update my question to clarify this. – Sverre May 25 '13 at 15:48