My two were cage mates before I got them, so they were used to each other already, but they still have their times where one gets on the other's nerves. They do get along most of the time now, but when we first got them, there was a time when they seemed like they were "fighting" frequently.
From what I have read here on this site, it is because they are establishing who is in charge. They will have some fights along the way, but as long as there is no blood drawn, then you should allow them to be together and work out their problems. You only need to separate if one has attacked the other so badly that there is blood drawn. It will make it worse if you separate them and then put them back together again. So my suggestion is that you take down the divider and let them get used to each other.
My husband thinks that our two piggies are fighting at night (he gets up at 4am and hears them making noises, rumblestrutting, popcorning and running around a LOT!), but from what he has described to me, they are playing and chasing each other around. Much of what you might think of as fighting is just mearly how they communicate with each other and normal behavior. Ariel probably didn't do much of that when she was alone because she didn't have another piggie to play with or another piggie to annoy her. Just think of them as sisters who will occasionally have an arguement, but that doesn't mean they can't live together!
