Two Cats Mounting Each Other?

I've only been able to find information online about one cat dominantly humping another, but I have two neutered male cats who hump each other in equal amounts! They're from the same litter, and are a bonded pair. One is dominant over the other, but they both take turns mounting each other just for about a minute at a time. Neither cat seems to mind much when it's their turn to be the "victim," with a benign facial expression and purring.

I watched the less dominant cat bat the more dominant cat away just now, which instantly stopped the behavior - and then they groomed each other right away and fell asleep cuddling. Then the less dominant one suddenly woke up, seemingly startled and a little trance-like (gave me a weird "I'm looking through you" stare with wide eyes), and then tried to mount the more dominant cat. I intervened before he could, distracting and then giving both cats some petting and attention once they were separated. They are still sleeping next to each other now.

What's up with them taking turns, and not being particularly aggressive about it? Is this a stress reaction? Again, they don't seem to mind, but I've also never seen them do this before (they are both 11 years old). They've just had to move with me to a much smaller house, so I imagine a lot of this might be spatial. I've read some suggestions to place food and water further apart, add a litter box, and add more toys to reduce stress and boredom during the day. What more can I do to add space or interest in a very small house, if that is indeed the problem? And is it likely that they're doing this when they're home alone during the day (usually from 8:30 AM to 3 PM)? So far I've only seen this happen after about 10 PM.