Hey there betta girl :) a half gallon split is .25 gallons of water each. there is no way you can fit a heater in there, betta fish are tropical. 100% of the water will need to be change out everyday, due to ammonia build up. it'a like living in a room which you pee and poop in and never get changed out. We are all experienced betta keepers and hope the OP will be on to a good start. how ever you keep your fish is a personal choice. But I must ask that if you do keep two fish in a divided half gallon and intend to continue doing so without regard to the tropical environment they need. please refrain from posting so on this forum. it will incite a lot uneccessary anger.
Companies which sell these tanks are all about profit. they could care less how much room a betta really needs. there have been companies selling ipod sized tanks. just because they sell them, doesn't mean it's the right things to do. I can't comment on how you keep your own fish, but please do not lead others astray. A 2.5 gallon tank is not expensive. they can be found for $15. and if you are really that unable to buy a tank, a rubber maid cotauner will do the job.
If you are looking to have a cheap fish just for decoration... I hope you will change your mind.
A fish will not only be healthier but will show a much more active behaviour when in a proper sized heated tank.