My PK stays in a five gallon all by his lonesome with live plants. For a Betta I think this is a little closer to a paradise. Keeping two male Bettas next to each other like that would probably be stressful to them as they will view each other as intruders.
I agree with you. Maybe I'll get a piece of black acyclic from lowes and glue that in. Although they get pretty accustomed to each other after awhile.