Not sure if his refusal to eat is due to some underlying but not yet revealed health issue, or it could just be he's still stressed in his new home, esp. as he can clearly see the other flowerhorn.
Also each of their compartments isn't so big, you're going to need bigger homes for both in the near future, they grow pretty fast at that stage.
Insofar as the other fish possibly stressing him out maybe you could cover the glass partition, e.g. slide in a piece of colored plastic future board (looks like cardboard but made of plastic) so they can't see each other.