putting two males together is a bad idea for most cichlids unless there is alot of space.... much more than you think they need.... for them to claim as their own. If you get a female and they begin to breed, the barbs will most likely get killed.... cons are notoriously aggressive when eggs or wigglers are present. For example, back a year or so, a TFF member had a huge 375G tank with an oscar, a pacu, and some other large cichlids (i dont remember the exact stocking), and when a pair formed from his school of 6 or so cons, they dominated the tank, keeping all the other occupants in a far corner and charging after them if they moved too far from the corner.
And as sheilaf said, 20G is too small for a pair of cons unless breeding, and if they do breed, it will be too small for them and all the babies after only a few weeks of growth. Either upgrade the tank size or just wouldn't recommend getting another con