Aaarrgh... of course, they don't just fight over toys. They fight over stones, sticks, the Argos catalog, plates, mugs... I try to get them to take turns, although they are allowed 'special' toys they don't have to share (mostly stuff they bought with their own pocket money). Sometimes I resort to taking item away from both and putting it on a high shelf, if they really can't stop fighting over it.
Same as Mrs Chop really. I havce two DDs who share a bedroom and a play room. We state that whoever is given a toy plays with it first, but then it is a communal toy they share and whoever has it first gets to play with it. If they can't share then the toy gets taken away from both. The eldest (4) does have a high up shelf where she can keep things she doesn't want her little sister to play with. On the whole, it generally works as they both like to play with each other's toys just as much as they play with their own!