Female rabbit is actually a male

I recently bought two Netherland Dwarfs who I were told were sisters and are now around 12 weeks old, however, this morning I noticed that one was trying to hump the other so decided to check the genders just to be on the safe side and discovered that one of them is in fact a male rabbit.

I plan on getting him neutered in a week or so but I’m unsure what to do in the meantime.

Should they be kept separate? Or would that upset them as they’ve been together since birth? And if I did separate them how would I go about introducing them again?

All advice appreciated