In my experience I would never put two males together, ever, unless you want to use dividers for individual compartments. Using dividers would be the best way to house multiple male bettas provided that you take precautions to properly glue the dividers down.
A sorority of seven to nine female bettas is perfect for a 20 long, but they take time and patience. If you're a betta/aquarium novice you might want to research before attempting a sorority since females can be as aggressive as males (if not more!). A sorority is wonderful though, and well worth the effort.
I hope this helps a little, and don't be afraid to ask more questions!