I don't think betta splendens are anymore "natural" than my dog is. Sure, she COULD be a wild animal, and is descended from one, but I wouldn't treat her the same way I would a wolf. Betta splendens are NOT the wild betta. I seriously doubt they could survive in the wild long-term.
That said, a big enough area would probably let them react differently than they would in a smaller tank. And there is no other male, so no one for the male betta to have to compete with for breeding rights to the two females. Interesting experiment.