I, personally, would use the 3 gallon instead of the 2 gallon for one fish, keep the 2 gallon in reserve as a hospital tank/if you find another fishy friend that calls to you, and try to keep whichever tank isn't heated as warm as possible until you can afford a second heater. I've attempted dividing KKs, its NOT easy, if possible at all(I was unable to figure it out xD).
If you can, keep the unheated tank next to the heated one or by your computer(if you have a desktop)for a little extra warmth.
That's just my personal opinion though. If your up for the challenge of dividing the KK as opposed to waiting for another heater, then go for it. So long as you keep up with your water changes, I don't see a problem with it as a temporary housing option.