Are they doing any bad damage to eachother? or is it just a case of locking mouths?

I had a pair of Red Terrors who were fine when in a tank with other fish, they were never apart from eachother. As soon as I moved them into a tank on their own the male turned and beat the hell outta her. So I took her out and re-introduced her again when she recovered, he did the same. So I tried it one last time and he did it again. I've had to keep him in a tank by himself now and the female is in with my Cubans. So the fish can turn on eachother just like that.

I guess just try stuff like move around aquascaping or try putting a divider in the tank for a while and keep them apart then re-introduce later.