Looking for advice to stop 2 female rats fighting

I have 2 female rats from the same litter, I've had them for a bit over a year. Sometimes they fight but usually the worst injury they cause is a little cut that heals easily. But it's started getting worse and this morning one of them had a cut so bad I had to take her to the vet for stitches. Does anyone have any advice on how to stop them fighting? Or at least to lessen the fighting? I just want them both to be happy and healthy