Hmm. It may not be fin rot but it sounds pretty close. I would suggest using a water conditioner such as API Stress Coat; it contains ingredients that will help healing.
Two males in a 1/2 gallon means you definitely want to change the water 100% every day. They'll get more stressed with dirty water than they will from a few minutes of water chages.
Ammonia is going to build up very fast so I don't advise this setup for more than a few days at the most. You CAN keep two males in a divided 5 gallon easily, though. You can also fit a heater easily into a 5g. I don't recommend dividing anything smaller than a 5g, though.
Both guys can probably benefit from the same treatment right now so if it's the most convenient for you, you can leave them together. You'll want to get some aquarium salt at the pet store and dissolve slightly less than half a teaspoon in a little bit of water. Pour that into their container with them. When you change the water every day, redo the salt treatment. Keep this up for a few days or until you get them a bigger tank. Once in the bigger tank, you can post its size and I'll work out new dosing for you.