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!”