How to auto-complete with comma-separated citations?

(This is an extended version of my previous comment.)

I'm not familiar with TeXMaker, but TeXstudio does the desired thing. That is, in TeXstudio,

• auto-completion for comma-separated citations in cite{...} is supported,
• if package biblatex is loaded, then that in parencite{...} is also supported.

Moreover, suppose you have newcommandmycite[2][]{cite[#1]{#2}}, then you can make TeXstudio's auto-completion support mycite by adding

mybib{bibid}#c
mybib[post note]{bibid}#c

to a .cwl file under TeXstudio's config path and mark it autoloaded in Configure -> Completion.

Related TeXstudio docs:

• What are cwl files good for and how do they work? - wiki
• 4.14 Description of the cwl format - user manual