My two cats have started fighting?

the only reason is because in cats, there is always a greater male if the population is males. don't keep them away! they will feel very lonely unless u keep them away with another female cat. even though they have been neutered they will fight for more supremacy. don;t give either of them more attention. this may lead to fights that may kill 1 of them. just flick both their ears hard every time they fight.the more they fight n the more u flick, the greater the resolution n this will stop the conflict. flicking their ears is 1 way of taming them.