To begin with, you won't be able to house 2 male nic's in a 55. I tried it, and had to move one out. They are relentlessly conspecific aggressive, and the weaker fish will die if you leave it there too long. I currently have a single male nic sharing a 75 with several male Victorian Hap's (they're about evenly matched, though the Vic's aren't as hard on each other as the nic's were), a pair of chanchito's that nobody bothers and vice-versa, and a group of Synodontil multi's.
Incidentally, both the Victorian Hap's and the nic killed off their females w/out producing any fry. The Vic's I could understand, sort of, because I had 6 males and only 2 females. The nic's were 1m/3f , but the males are just outright psycho IMO.