If you want to share the game and only buy one copy, I'd think your best bet is to buy a physical copy. You wouldn't be able to play at the same time, but I don't think that'd be possible regardless of physical or digital. My son and I have the same problem with PS4 games -- Sony at least lets you share physical copies, not sure about Microsoft. Otherwise when it comes to digital copies, you can only share a digital copy on the system that is marked as your primary console. So anything digital games I get are ones he isn't going to play.

Long story short, the only way you guys are going to be able to play at the same time is if you have two copies.