This is entirely up to Microsoft, and sony, Bungie will have NO control if either says they don't want that to be an option. And the way each companies marketing team will see it, making each network separate will increase sales. This is because people with both consoles may buy it twice, to game with friends that only have one. It could also be said that more people in general will buy it if cross-console gaming is available. I would say there is a 30% chance that cross-console will occure.