One thing I would try is to put the two fish in their own tank and have a divider (something that the fish can see through) in the middle, but cut it short so that it is about 1-2 inches above the substrate. Put something the fish can spawn on in underneath the divider.
Usually, the temptation to breed becomes too great and the female will lay eggs while the males fertilizes them from the other side. That is usually how flowerhorns are spawned, and they are very aggressive even when in pairs.