My girls do this, too. Clare will relentlessly mount Guenevere all night. I always feel so bad for Guenevere! I hate to watch it. But, luckily, as persistent as Clare is with her dominance, she's also gentle thank goodness. She doesn't get all Miss Dominant every month, but every once in a while. I always check Guenevere the next morning, and just keep listening carefully throughout the night for any actual fighting. But no, I don't remove Clare. It's part of their natural behavior to display dominance. So, as long as nobody gets hurt and they don't fight, then they should be fine.
But do watch for wounds and fighting...if that happens then separation for the night might be inevitable.
How long have they been cagemates? How old are they?