Our guinea pigs do the same thing.
Any other time and they don't get so close together. In the vet's office they take turns crawling under each other.
"Mounting" is dominance behavior among guinea pigs regardless of sex. If a guinea pig climbs on top of another one, it's saying, "I'm the boss". So it's kind of amusing in a sad way to see the behavior change from trying to establish dominance to trying to force the other one to establish dominance.
"You're the boss!"
"No, you're the boss!"
Also it's not good for the cats either.
The stray cats I've known who found indoor lives never want to go out again. It's the spoiled, pampered cats who incorrectly think they're tough who want to go outside. The cats who've seen some shit know inside is where it's at.
Damnit! Felix got into the replicator again!
When you're around a cat, stare at it. It will leave you alone.
Just don't stare too hard, or you'll convince it you're such a threat that it must kill you.
It's serious. Looking at a cat is interpreted as threatening. To be non-threatening, you have to look away and even close your eyes. By ignoring the cat you show that you aren't a threat.
That's why cats always approach the person who doesn't like them. That's the only person looking away, so it's the only person who is signaling that they aren't a threat.
We need another banana for scale.
Just cover the entire floor with a layer of kitty litter.
Another story, same cat:
I was installing a window unit air conditioner. Step one, pull the unit out of it's shell. Step two, mount the empty shell in the window. Step three, install the unit back in the mounted shell.
Later as we were enjoying the new coolness of the house, we discovered the cat was missing. I figured out pretty quickly that the little pain in the ass jumped through the empty shell and out the window when I turned around to pick up the unit and finish the install.
Both our cats were indoor-only cats. The older cat would have been able to feed himself, but he also wouldn't have been stupid enough to go outside in the first place. She in particular did not know how to survive. She got her ass kicked by a beetle once.
We were leaving on vacation the next day. We printed up signs, put them around the neighborhood, and I figured she was gone for good.
When we got back, there was a message on the answering machine that someone had found her, and put her inside our enclosed side porch with a bit of cat food.
We checked, and she was there on the porch. Skinny, dirty, and stupid, but alive.
That's pretty lucky. Years ago I was adding a ceiling fan and opened a wall in the attic to run the electrical line.
That led to meowing coming from the walls. Luckily we were able to convince her to come out an access panel in the other end of the house.
My childhood cat, Gimpy, used to steal rolled up socks. He'd carry them around, chirp at them, and give them drinks of water (by dipping them in the toilet).
In an excellent example of the lengths a cat will go to in order to fuck up your day, one of my cats jumped up to the top of the post at the top of the stairs and puked down the side of the post from his perch.
It was honestly impressive in a what-kind-of-sociopath-would-do-that way.