If they are the only 2 fish in there, thats part of the problem. No other distractions, and they both want the whole tank, the less dominant one will probably end up getting killed.
If you want to keep both, you could try taking the more agressive one out for a couple of hours, add some other fish, a shoal of rosy barbs or gold barbs would be perfect with firemouths (or some bleeding hearts if you want to keep your stocking from similar geographical area), and a couple of reasonable sized catfish like hoplos or similar, and then move the decorations around and put the agressive one back.
Failing that, you could get rid of the aggressive one and add maybe a female convict or perhaps a rainbow cichlid instead, as the battle for dominance will be less amongst different cichlids (not two of the same type).
