Also there is this theory in some cultures, that twins share a soul,because of this in those cultures one of the twins is killed, they say that if they did not do this the twins themsleves would battle for the soul throughout their lives eventually one coming on top of the other and beating the other from it. (Or something like that - hey its been a while since i learned this but it comes down to that more or less)
Because of this i am thinking it will be Jaime, especially since they have had such a close bond with each other, Jaime till now seemed more attached to Cersei (thus atm cesei according to theory was holding majority of the soul ---> holding power over Jaime), in the last books that is changing and it is now jaime who is bassically holding power over Cersei, she bassically needs jamie to come and rescue her.