You can give your flowerhorn a tank divider as a mate. After breeding and keeping these species for year or so, I haven't found anything that mixes with them unless you raise them with other fish since they were babies.
They are fine until they mature. Then they get nasty, IME. Some males are just born that way though. Texas cichlids do well with them. And my bichir does well too. My jacks got bullied and so did everyone else, except my 21" G. tiles. RD do well from what I hear... and the fry look pretty awesome if they happen to mate... FHs are whores and will pair off with many many different fish species. Honestly, every fish is different... trial and error in most cases is the only way to know for sure.
That is incorrect info, it all depends on the fish they all have different temperaments. It also depends on the size of the tank space and the way it is set up. If FH don't do tank mates than why and how do I keep 2 males, a salvini, 3 large African's, an eel, a leparanus, and a cat all in my aggressive community tank for years now? You just need to know what you are workin with that's all I will add please do not try the two male thing without knowing your stuff and or fish this took many trial and errors for the most part unless breeding only one FH per tank is a good idea, but other tank mates so long as the conditions and fish are right you can pull off easily just got to know your fish