Just to chime in on the off chance it helps: my two girls also got along fine but would still get into occasional spats, mostly over food. Only one of them would try to hump, but they didn’t have an obvious “boss” between them. I did notice they would sometimes guard the litterbox from each other; usually one would already be in the box and when the other would come up to eat, they would kind of hunch down and pin their ears flat and grunt. In my set-up I use two XL cat litterboxes (~22 inches long) next to each other in one corner of their regular area, and they would kind of rotate between them– sometimes sharing one box, sometimes separate. At one point they started getting more aggressive with food and just seemed to be annoying each other more in general, so I had to mix up the feeding routine and moved one of the litterboxes to a different spot, a good 6-8 ft away. I figured it would be harder for one bun to guard two boxes at once. If I caught the behavior I would coax the bun that was pushed away to the unoccupied box. After a while (maybe a few weeks) they got way better about it. You might not have the same amount of space, but I wonder if rearranging their area occasionally would kind of break the routine a bit and reduce the guarding behavior– I noticed you said the peeing would stop for a couple of days after you reduced their space or added a second litterbox.
If it truly is territorial behavior, I wonder if your girl is feeling insecure. Outside of the litterbox behavior, there might be other stuff you can do to kind of reassure them and reinforce their bond, like: feeding treats simultaneously (instead of one bun before another), or petting them when they shove their faces against/under each other, waiting for the other to groom them (my girl Mischa would escalate to nipping if she felt she wasn’t getting groomed enough, which could sometimes cause a chase). One thing I also do is just verbally praise them when they’re doing what I want: taking appropriate turns grooming, actually sitting IN the litterbox and not just outside it. I can’t say for sure that it makes a difference, but I feel like it does!
Good luck and hope it works out!
