Suzanne - I swapped their cages every night so that every time they went home they had the other pairs scent in with them. I put them in a small cage (with nowhere to hide from each other!!) and just kept a hand on the one with a tendancy to nip (Jessica!). When she got nasty I shouted "NO" at her and clapped, and she soon stopped. Did that for about 15 minutes at a time every night. Now I leave them out all day together and they are fine!
Hope theres some handy tips there?!