depending on which species you where planning to mix.
I have mixed Phelsuma Modesta leiogaster 1.1 and Phelsuma Breviceps 1.1 into 1 tank. I know a man was keeping the same as I do including a pair of Phelsuma Mutabilis. they even breed regular, not much but they do well.
but i more agree with Sander. don´t mix up phelsuma´s into one tank.
and do not cross breed !!!!!
regards,
jeroen