sam,
yes, they do fight at times. afterall, they themselves are kids. i always tell them if they don't share, whatever they are fighting for belongs to mummy. so now to prevent whatever is gone and becomes mummy's, they will take turn and share.
#1 will be playing with it for less than 1 min, #2 will be standing beside and say, ok, its my turn already. then #1 pass to her and it becomes #1 to say, ok, mei mei, its my turn. then #2 pass back to #1. this can go on and on till either one don't want to play.