Splitscreen USED to be a command in 1.09.4. You'd just do "splitscreen on" and Tails popped in with the screen splitting in two, unless you had done this already and turned it off, at which point it's a green Sonic that can't move.
I haven't seen any sign of the command or a replacement for it in 2.0, however. It was probably removed due to being able to use MAP immediately afterward without setting off the cheat protection.