some say it's not ok to put a female and male together in a divided, due to the possibility of mixed phermones and the female becoming egg bound. However, there have been females who got egg bound simply from seeing a male. I think any mix should be fine, as long as you are not putting in another betta with a male.
As for signs of good health, other than having no signs of illnesses, they would need to seem active and respond to you. There are certainly placid and slow moving bettas who are healthy, but I would go for the more active ones who respond to me.
When shopping for a new Betta, what are the signs of GOOD health? I know the signs of disease, but not for really good health.
Secondly, does it matter what gender the two Bettas in the divided tank are?
Healthy bettas will NOT have clamped fins, show clear eyes, flare at their neighbor(for some bettas), show their colors brightly and do not have many fin ailments(no tears, rips, fin rot). I have heard if you put one male betta and one female betta in a divided tank the female can give off pheromones used for mating and stress the fish out. Males in a divided should do fine.
Anybody please feel free to correct me if i am wrong, but i think the second answer is true.