If they are fighting, then you'll need to separate them before one gets injured. Also, any fighting now will decrease their chances of re-bonding them.
There are a number of reasons that this could occur. If one is not feeling well, it can trigger a fight. The one not feeling well may be grumpy causing him or her to lash out. A change in their environment, the addition of a new pet, or if one rabbit was taken somewhere without the other one (to a vet, for example). Any of these can trigger a negative reaction.
Can you think of anything that might be different that could have triggered this? Also check over each rabbit well to see if there are any signs of a physical ailment. Are they both eating and pooping normally too?