I used to have 1 bun + 1 yan yan. It was ok to begin with but then the yan yan sometimes use misunderstanding as an excuse for not doing thing properly. She was taking advantage of the bun bun.
Now that I have 2 bun bun. They got on ok and sure they can't use misunderstanding as an excuse. But you have to prepare that they talk and gossip quit a bit more.
I think all you have to do is to give simple and direct instruction of what their tasks are and be fair in treating them.