I guess if they weren't related, then it'd be ok!
No need to worry about unwanted litters.

Prehaps it is a dominance issue.
Or is something in or around their cage setting off chemicals which are
interfearing with their hormones?

Are there wild rats in the area whose smell might be "turning them on" -
so to speak?

I've seen male rats do this with each other, but then, that's men for
you (ha ha) - No, alls equal in love and war -
But the explinationfor males is dominance.

Prehaps if they are young females, then this is all part of their
developement - they just have a stronger urge than other female rats.

I have seen female mice act in this way, once when I had a heap of them
handed in to me, they were crowded together, with no amusement like
running wheels etc...
Two of the females, one more than the other, had taken to 'rooting' the
other females, moreso the younger or newer ones.

Yet, I am at this time unable to explain this any futher.

Prehaps buy two unrelated females, seperate these two, give them a new
partner each and ...well...problem solved ( unless ou were hopeing for
grandchildren)
cheers,
Rebekah blackwolf.