A 20 gallon is perfectly fine for 2 frogs. A 20-Long is better than a 20-High though as it gives the frogs more horizontal swimming room. As far as males and females, get what you want. I prefer females only, but that is just me. Feeding can vary. You will want to feed froglettes at least daily and adults every 2-3 days. Get them on reptilmin as soon as you can for their basic diet, then add in things like store bought earth worms and thawed frozen krill , table shrimp and mysis shrimp as healthy treats. Stay away from brine, daphnia, and blood worms for various reasons. Though I am sure many people will disagree with that. Also avoid any kind of freeze-dried food at all cost.
Make sure you are patient and cycle your aquarium before you purchase frogs.