Are they spayed? if not, that's what you need to do to help them; get them spayed.
However, mounting is a part of a rabbit's behaviour and communication (dominance) so you can expect to see it generally in bonded pairs, but it is important to get them spayed to ease it as much as possible.
If it is getting 'tetchy' between the two of them then you may need to separate them an rebond after they are healed from their spays.