2 Bettas sharing 1/2 gallon of water? YIKES! 2.5gallons per betta is the minimum recominded water volumn per fish. I've heard some people can pull off 1 in a 1 gallon. How are you keeping the amonia and everything in check? Especially after feedings. And does it have filtration?
Now I'm not dogging you hun. Those fish survived much worse and at least are getting cared for now. Take good care of them, do frequent pwc (I would scoop out 30-50% every morning after feeding and another smaller 1 in the evening without cupping them), and work towards something that the will work better.
The price for a bigger setup isn't bad. Walmart sells plane 10 gallon tank for $14 and an unlighted genertic cover for $7. You can use a desklight with good cfl bulbes to light it until you could put in lights. Filter would be optional (but I do recomend). Heater to keep it proper temp (unless their water never drops below 76 degrees). Termometer. And 2 dividers (diy or bought)
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