My rat attacking the other.?

I have two rats(both females). I've removed them a few months ago from eachother, because they were fighting pretty badly. Recently I tried to put them in the cage together, hoping that their seperation would've helped. I woke up the next morning with the smaller one out of the rat nest looking very frazzled. Ever since, I haven't put them in the same cage, but they do see eachother when I take them out every night, though the larger one always tries to get at the smaller one. I used to stop it everytime, but now I'm thinking it's a sign of dominance and I should let it go. What do you think and do you think it would be okay if I left them in the cage overnight?