Is it staying at the surface but in a corner or behind a filter Stem ? That's fairly common, its just nature doing its thing.
You put two fairly aggressive fish in a tank and the Dominate one beat the heck out of the weaker one. With no place to go (obviously because hes in a tank lol) he will reside himself to the top corner as a sign of submission. He will stay there till he eventually dies from stress.
Dempsey's are VERY Aggressive Cichilds, in order to keep two together you would need at least a 90 gallon tank or bigger. Even that wouldn't guarantee they wouldn't still go at it, fish preserve areas much bigger than a tank as "Their" turf. It also depends on the temperament of the fish, sometimes if you get lucky and get a couple of placid fish they might be able to live together.
You got a true Dempsey though, hes not gonna be able to have any tank mates besides Plecos or Catfish. I would take back the beat up fish now and just trade him for one of those if you can. By leaving him in the tank your putting both fish at risk, If the stressed one doesn't die outright and develops a illness because of his lowered immune system. He may in time infect both fish and that wouldn't be something you would want. Leaving him in that tank would be cruel really, not much of a life for him. I know you where probably told by the "know it all" at the fish store this could be done, but that's an exception, not the norm. The norm is what your seeing right here, one fish dominates and stresses the other.
The only way you can have two Dempsey's live together is if you get a male and a Female Pair. To do this you would need to buy 6 young 2inch fish and let them grow up together until a pair forms. Then stick them in a 90 gallon and sell the other 4. If you really wanted to have two of them that would be about the only way.
Good Luck
