Female cat keeps biting other female cats neck

My boyfriends two female cats (Micro and Yaya) get along super well. Both are spayed and they spend the vast majority of the time with each other, always cuddling and grooming each other. I've never been able to make either purr as loud as they make each other purr.

Anyways, despite all of this, I've also noticed a funny behavior where they bite each others necks a lot. Not hard, but sometimes they'll nip it while other times they'll assume a mounting position (bottom cat never struggles but obviously their scruffs are being held, but even after they're released they just sort of chill there and return to cuddling). Both are spayed and my boyfriend and I joke that they're lesbians but unironically they're incredibly friendly and close with each other and engage in this behavior in pretty equal amounts only towards each other (no other cats). Can this be a dominance thing even if they're so close and comfortable with each other?