It looks like things are back on track, with a couple minor differences.

After their spat, I reassembled the 2nd NIC cage (sigh). For the first couple days after that, we went back to bonding sessions in the bathroom, and for the past 3 or 4 days, we've been bonding them in the xpen in the living room. They're doing well - snuggling and grooming each other - although I still have to keep an eye on them. Digby doesn't seem interested in mounting Yoshi anymore, but Yoshi does often stick his nose in Digby's hindquarters. I think Yoshi is nipping him, since after a couple seconds, Digby gets annoyed and turns around. Last night Yoshi tried mounting Digby again, which never ends well. He can't really hold him down, since Yoshi is so much smaller, which means that Digby just turns around, and they both go after each other's hindquarters, spinning around. I think Yoshi still really wants to be the dominant one though. He's definitely more comfortable now, as he often flops next to Digby - I hadn't seen him flop at all before, unlike Digby, who's a chronic bunnyflopper

This morning, Yoshi was a little weird. The two of them shared some veggies and snuggled a bit. They were both in the litterbox, then Yoshi hopped off to the opposite corner and sat down. After a little while, he thumped. Digby hopped over to him and snuggled up. Then Yoshi thumped again. Digby stuck his head under Yoshi's, and Yoshi started grooming him. Yoshi thumped again, but kept grooming Digby! I guess Digby wasn't the thing that was annoying him (might have been the sound/vibrations of the heater or dryer from downstairs).

They started fighting again while bonding last night. I didn't see exactly what started it, but they were in the litterbox again. I think next time we're going to try in a new area, maybe the kitchen, without the litterbox.

Beka, I'll try to keep Digby calm so Yoshi can get the humping out of the way, but Yoshi usually starts by nipping his backside, and it usually doesn't turn into humping.

Does anyone know any bonding experts who aren't here on BinkyBunny? I'd love to get some advice from someone with experience bonding 2 males.

The bonding is... still going. Each session, they get into at least one little scuffle. But they also groom each other each session. I think I'm going to give them a day or two off, then start again with either a car ride or washing machine ride.

They do eat veggies together peacefully, but they seem to be getting slightly more aggressive over the past 2 weeks. Sometimes the water bottle helps, but not always. Should I spray one as soon as he nips the other's backside? Often the one being nipped will just turn around, sniff the other's head (or sometimes groom them), and they'll be fine.

I haven't been putting a litter box in with them for bonding sessions, since that was usually where the fighting started. Once I feel they're doing well enough to leave them in there for an hour or more, I'm planning on putting two litter boxes in with them.

We're in neutral territory - an xpen in the living room, which they've only ever been in together. They seemed to do fine sharing the litterbox before their first fight, but it's also where their two biggest fights started.

I want to thank everyone for all the advice on this thread (and others). I definitely would have been much more stressed out without this great resource and community.

I tried putting them both in the litterbox. I put Digby in first, then Yoshi. Unfortunately, as soon as I put Yoshi in there, Digby growled and lunged at him. I tried petting them both, but Digby wanted to get at Yoshi and Yoshi just wanted to get out. It's a litterbox I keep in their exercise room that they've both used.  Next time I'll try cleaning it with vinegar first to remove any scents.

I put up a dividing wall in their exercise room so they could both be in there at the same time and see each other. It seemed pretty sturdy (2 rows of NIC grids attached to chairs on each end), but tonight they found a way to get through it. There was fur all over the floor. They must have fought quite a bit, but when I came in they were snuggled together under a cardboard tent and blanket I had set up in there. I brought some veggies in there, and they both ate, but when we had a chance to examine them, we found a bit of a gash on Yoshi's shoulder. It didn't bleed at all, but the skin was broken a little, so I put some triple antibiotic ointment on there. It's about 1.5 inches long and less than quarter of an inch wide.

The divided room seemed to be working really well until now. I think being in sight of each other 24/7 really helped, there was no aggression at all during the bonding sessions.

I feel terrible - incredibly stressed out and sick to my stomach. They seem ok - both are eating veggies and hay. I don't even have time to take them to the vet tomorrow.

I sure hope they've sorted it out amongst themselves. I don't know if I can take another night like this. My heart skipped a beat when I came in the room and saw that the divider was down and there was fur everywhere... then I saw them snuggling together.

I think I'll do a very short, very cautious bonding session tomorrow if they both seem to be ok.

I really hope so. Unfortunately I have had to hold off on bonding sessions, since Digby is very persistent about trying to groom Yoshi's wounds, and I'm worried he's going to make them worse. Yoshi is healing well though, so hopefully I can start bonding them again in about a week or so. I've reinforced the barrier in their playroom, so they're still in sight of each other all day. Occasionally Yoshi will run back and forth in front of the barrier, like he's trying to taunt Digby, but they also lie down next to each other and groom themselves, so I'm hoping that they won't have to start back at square one again.