20 gallons is the minimum for a breeding pair, 2 unpaired fish in a tank that size is asking for trouble. I guess utube can teach me something I haven't learned in 10 years of keeping & breeding angels. Fully half of trying to sex angels is understanding behavior as a group, which two is not. 50% accuracy is just guessing, I can hit around 80% with a group of 6 or more provided they are approaching maturity.
Bottom line is what you should be adding to the mix is a larger tank.