We went from laundry basket, to bathtub, to bathroom, to exercise pen, and then finally a whole room. They didn’t even start grooming until the bathroom! I started only leaving (at the exercise pen stage) them alone when going downstairs, then slowly increasing the time I’m gone. I think if they’re in a pen together, and have groomed each other, and began relaxing (flop maybe, or just lying down), they don’t perceive the other as a threat, and you can start leaving the room for periods. I would not recommend leaving them if one is still territorial in the same pen used. But to me, you’ll just KNOW when it’s safe to leave them, if you have even a doubt, it’s not worth the risk.
PS; Overnight is a great bonding session! Try putting them in the same pen during the night, and sleeping next to the pen. This way, they spend loads of hours together, and you can easily wake up if a fight ensues.
The bonding with be much easier if you let them figure it out. This does not mean to just watch them fight, but if its a small nip, or mounting, or a little chasing and lunging, it’s their way of figuring out who’s boss. If it’s a full-on fight, separate them. Also, try having them eat together (social activity) or try a stressful situation.