Is it necessary to make a division in this way? My boys never seemed to feel the need, and nor did we, so we never pursued this distinction.
Your age gap is a bit bigger than mine. There are 22 months between my boys, and basically they don't have 'yours' and 'my' toys/games/books, they just have ours.
Certainly this was the case 100% when they were younger. They are 9 and 11 now, and do have a few things which are owned: they each have their own DS and games. They share the DS games, but the 'owner' has priority. Also they have some different hobbies and interests, which would identify the owner of a few items.
Apart from that pretty much everything is shared/joint-ownership, no matter who 'got it' for Christmas/birthday. All the toys/games/books are in the shared playroom (not bedroom) on shelves/in boxes. If you went in and picked up a toy at random and said "whose is this? you would almost certainly get a blank look.
When they get something for Christmas the recipient will get first dibs or whatever, but usually it will land on the 'common heap' (except for food!) and everyone gets a chance. If they get a book they get to read it first, but then the other will often read it afterwards.
I think this sort of set-up is unusual, but I'm not really sure why. It just makes perfect sense to us.
I think as they get older and want phones, ipods, etc. they will have more personal possessions, but it just hasn't seemed necessary so far.