Both the fur baby's are just trying to show who's dominate over the other, it's not sexual.
Im thinking but not sure that the two of them will work out their "learning who's boss" and move on and you will see less of the "humping action".
If not, then maybe some scolding can take place?
Dunno..
Schatzie has only humbed Annabelle which I've seen a couple times. I usually just say "knock it off" and she leaves her alone.