Is this a multi-player game? With two boys, I can't buy unless they can BOTH play at the same time.

Yes and no. Technically it is a 2 player game where they can play at the same time however when you play like this the 2nd player does not have a character on the screen. What the 2nd player does is use the remote to help the first player pick up items. So while they can play at the same time the 2nd player is so limited that they might as well not be on there. Nevertheless the game is super fun. We have 4 boys and they like to help each other play. I would still highly recommend the game. They would enjoy it more with their own separate files.

Which is better super mario galaxy 1 or 2? I bought SMG 1.

Hello I hate to admit it but I haven't played the first Mario Galaxy. I bought Mario Galaxy 2 for my daughter when it came out.I haven't had a chance to go back and play the first one. I highly recommend the Mario Galaxy 2. Thank you

Can a five year old play this game?

Yes, definitely the child can play without problem

does this game has a split screen mode ?

This game does not have a split screen mode. you can use a second Wiimote to collect and fire start bites for player one. it is not a true two player game but you can have a second player help player 1. Hope this helps!