Male and female bettas should not be kept together except for the brief period of spawning - and even then they should be watched closely. They may be co-existing at the moment but it could easily turn into a disaster at a moment's notice. Rather than upgrade to a bigger tank it would be better to get 3 smaller tank of around 5 gallons and move the bettas into one each. Then use the 20 gallon for other fish.
if you really want to upgrade, the best solution would be a 5 gallon for the male, then get some more females and move the existing females into the bigger tank at the same time as the new females are put into the bigger tank. That way you are not adding new fish into the current fish's existing territories, they are all new to the tank.