I only use tanks that small either for babies for a BRIEF amount of time before they get moved over. Or the 1/2 gallon ones as a QT tank (esp if they have SBD & have problems surfacing). But it's even hard to dose meds or even dechlorinator right if it's not mixed up in something bigger. My rescue w/SEVERE fin rot and some spinal problems is going to try and be moved into a bigger tank with "levels" so she can rest in between and hopefully the filtration won't be too hard on her. If not she'll be in a one gallon. With 16, I'm used to water changes everyday on them-I even have to keep a log of who got what when. All of my little tanks are heated (unconventionally, but heated). But 2 full grown bettas in a teeny divided tank is cruel. My adult males are too big to even really turn around, let alone swim.
Bettas-Juniper (male), Bentley, Cameron & Emerson (babies)
Horse-Champ (Quarter Horse/Thoroughbred)