We both use Keyboard(we dont have one each) and whenever i move or jump or whatever his character also does it. when he moves he doesnt control my char. I havent tried to see if it happens in other games.
EDIT: http://www.reddit.com/r/emulation/comments/18dyg0/how_do_i_fix_this_in_new_super_mario_bros_wii/ problem fixed, got help from here
EDIT: http://www.reddit.com/r/emulation/comments/18dyg0/how_do_i_fix_this_in_new_super_mario_bros_wii/ problem fixed, got help from here