I thought your girls were already spayed… I must be thinking of a different member.
Unfortunately, I have to agree with the others. It isn’t safe for them if they are humping the “wrong” direction. If they are biting each others’ genitals, this can lead to infection, or if they become violent during this, the injury can be much worse, possibly requiring surgery to repair the area.
I understand your concern about whether or not it is “mean” to separate them. Sometimes as the thinking, reasoning “parent” in charge, you need to make decisions that “the children” don’t agree with… but it is done for their SAFETY. You will feel much, much worse if your hesitation to act results in one or both becoming badly injured.
What is your timeline for their spays?
Another thing to keep in mind is that certain behaviors can become ingrained in them prior to fixing, and they may continue to behave in that manner afterwards if you wait too long to fix. And if they continue to have less than ideal experiences around each other, this is likely to affect their bond after as well.