Can you keep two flowerhorns in a tank?

Yes. you can keep two fishes in one tank as long as they are separated by a glass divider and the tank is 50 gallons plus in size.

Some of them are not that aggressive and do tolerate other fishes including other flowerhorns. As flowerhorn community tanks are kept by flowerhorn hobbist's along with other types like Parrot, Kingkong Parrots, Oscars as well.

A best way to achieve this is by getting flowerhorn fry(a lot of them) and keep them in a spacious tank with caves and places to hide in. Eventually the once that are most aggressive or dominating could be separated and we could have a bunch of flowerhorn that are tolerant to each other.

Another trick is to get keep a middle sized parrot (i would prefer to keep full size parrot just my opinion) when the flowerhorn fry are brought..Parrots play nannies to them and teach them some manners.

And the safest way to do so would be to put a glass/plexiglass divider.

Keeping more then one Flowerhorn together depends on the comfort level these guyz would develop around each other(even if divided coz if you want them to reach their true potential will have to provide them with what makes them comfortable).