I'm developing two programs for a project, one client-side and one server-side, where the client program is in python and the server is in java.
My question is are there guidelines (e.g. by github, subversion, etc.) stating that these two should or should not co-exist in the same git repo?
Since you'll generally be compiling and distributing them separately, I'd suggest separate repos. They are -in that case- separate projects.
One artifact per project keeps thing nice and simple. One build command per output. Having multiple projects in one repo means a complex directory structure, lots of build tool customisation (in the case of,say, Maven) and possibly complex build commands. It does mean -however- that any communication changes will need to be made to two projects but as client and server are in different languages you'd need to do that anyway.