Which genus of bricardi are we talking about? There are several different genuses with the same species name and some are much more suited to a 20 gallon then others. If you're talking about N. bricardi which is the most common in the aquarium trade, two of either sex are going to get too big for a 20 gallon tank. Males would definitely kill each other, and I believe you would aggression issues with the females as well in that small of a tank. Like HUKIT said, if you are limited to a 20 gallon I'd look at a different fish.
"Listen to some of these guys talk, and it's like they were born from their momma's belly with a fishkeeping encyclopedia in one hand and an API kit in the other" (unrevealed).