If you make new dividers out of acrylic and put .25" holes in them, the tank mates can have the entire tank while the Bettas stay confined. That's what I did and am quite happy. The only thing I would do differently black dividers instead of white since I have black sand. But if I did that, the light for the plants wouldn't reflect as well.
I have a filter in each section.
Here's the tank when I first set it up to give you an idea. It's a 20 long. It has Bettas, male Endlers and Dwarf Panda Guppies, Pygmy and Hasbrosus Cories, PFR, Yellow, Blue Velvet, Malawa and Amano shrimp, Dwarf Orange Cajun Crayfish and Thai Micro Crabs. Also Assassin and Nerite snails.