You turn on the other wii controler then you press 2 player on the game!
No you only play as Roxas at first then Sora, then King Mickey when he saves you when you die, then Riku when you save Sora from Xemnas.
I own Kingdom Hearts 2: FinalMix+ and you can fight Terra. He is an optional boss and he is unlocked only after you have beaten every other world. After you have beaten the other worlds, return to...
Use Common Sense. If you have Disc 2, and the game asks for it, put it in.
if you have final fantasy 7 for playstation 1 then it should work on your playstation 2