One should be OK in a 20G, although most references I have read call for 30G for one. With the possibility of keeping a pair in 30G if you are lucky enough to get a pair. Two males will fight.
To increase the chances of getting a pair you should pick two with as great a difference in size between them as possible and introduce them to the tank at the same time. Sort of like you would with clownfish. Only in this case the larger is usually the male and not the female.
I've also read that in some species of Centropyge angels there may be a slight difference in the appearance of the soft dorsal and soft anal fins between males and females.
The male may have soft dorsal and soft anal fins that are slightly more pointed (less rounded) than the female.
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