Multiple Local Players not allowed, but should be already supported

[Georg Friedrich Schuppe]

Describe the bug
Looking at the SessionBuilder, I just realized that we do not allow multiple local players.
I am however quite sure I implemented this functionality ages ago. The endpoints are set up to handle more than one player per endpoint, which requires more than one local player one at least one of the clients.

Expected behavior
Adding more than one local player should be allowed. In the best case, this check can just be removed.

What to do
Write a test case for this and maybe even extend the example. See if this actually works.

[Georg Friedrich Schuppe]

Turns out, the check is completely non-functional and cannot be reached.