I have a group of four male coccinas in a 5 gallon tank, but there's a 2 or so inch leaf litter at the bottom they hide in, and they are more talk than fight.
However, no way would I recommend keeping two males betta splendens together unless the tank was at least 30-40 gallons, and so densely planted you couldn't see from one end to the other.
Bettas tend to hunt each other out, even in larger tanks. I have had troubles with my divided tank recently in which my male kept getting in with my extremely docile female. Even with lots of hiding places and a full 20 gallons, he would chase her down and managed to put some nasty tears in both her dorsal and caudal fins. He had no intention of spawning, he just wanted the whole tank to himself.
Two males would be even worse I imagine, especially two unrelated and adult males. Plus, the daily stress of co-inhabiting with a possible competitor and rival would make it not worth the effort.