Convicts and Kribs grow to around five inches and would in my view ,be too large as adults for 20 gallon tank.
Add to this possible fry, and the tank (20 gal) would be difficult to maintain water quality. No other fishes could be kept with the convicts if male and female spawn, and you cannot GIVE babies away. (no demand)
If two male convicts were in such a small tank ,one or the other will be beat up on as the fish reach sexual maturity.
A pair of Kribs might work but again ,numerous fry would make tank maint critical for the fry survival as well as the adults and often ,male kribs are nasty with females that don't wish to spawn with a particular male or as often as the male would like. Females are often nasty(mean) with smaller males as well as are many cichlids.
A pair of German blue rams could work if your water is soft and acidic and they require warm temps that other fishes may not enjoy. Rams do poorly in temps much below 80 degrees F and they do not do well in hard water or pH values much above 7.0. In the long term.
Apistogramma or shell dwelling cichlids as mentioned,,,would be more suited for such a tank providing that care was taken to ensure your water chemistry meets their requirements and suitable tank decor is provided.