I agree with Devon here.
There are very few bettas that will live together, even females. Males should not be put together. Even if they are females, 2 is not the magic number and it very rarely works! I'd get some tanks for the bettas to be separate because angels aren't always little angels either! Ok, so it's worked for you so far... But one day you're going to turn on the light and it won't be so pretty!
Ok, done with that part...
Take the betta with dropsy out and put him in a quarantine tank to treat him for dropsy. It's not contagious, so you don't have to treat the whole tank.
While you're getting a Q tank, get another tank for the other betta!
Dropsy could be stress related or it could be a secondary infection to something that's already happening with your betta. Do his (or her) gills look inflamed? Is his tail shrinking or does he have fin rot?? Any interesting, out of character, things going on with your betta lately? (Suddenly I sound like a psychologist!

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