Betta come from the shallow, often acres-wide rice paddies of Asia. They do not come from puddles.
Males are solitary unless it's breeding season and MUST NOT be kept with other males or females. Females may be kept together.
However, you have too few female betta. They are both stressed out because they are constantly harassing one another for dominancy.
Females must be in groups of at least 3 so that their dominating tendencies do not spread out so far, especially towards other fish in the tank.
I would HIGHLY recommend that you go out and but at least 2 to 3 more female betta and then when you get home with the, take out the two you already have.
Acclimate the new betta and introduce them to the aquarium then shortly after, re-introduce the original two as well.
You may see some bantering as they determine who is their dominate and after that, you should see a significant decrease in the number of fish being killed off.