Right so before you eat my head off, it was an accident, they have now been separated and you should not try this at home!
So, I just got home after a 2 day drive from Missouri to Massachusetts with ALL my animals: 16 bettas, 1 Bearded Dragon, 1 Water Dragon, 1 FAT cat, and 2, 4" goldfish. I was pretty proud about getting 1 30 gallon and 5 10 gallons into my Camry lol. We didn't get home until 9:00 last night. I was sooo tired but of course, all the animals need to be set up before I can go to bed! To my dismay I found that my dividers don't fit in the 20 gallon long (duh) I had planned to house my 5 male bettas and 2 unsocial females in. So I broke out my "betta barracks" which I HATE and don't suggest anyone ever uses... and placed the 5 males in those for the night and allowed the 2 females to get free range of the 20 gallon. I floated live plants so they covered the entire surface of the tank, ran a strong filter and heater.
I woke up this morning to 3 of the 5 males escaped from the barracks (surprise surprise) but completely at peace with one another in the well planted 20 gallon tank with the 2 girls... I was absolutely astounded. I'm not sure how long they were together but there were no torn fins aside from a nip on one of the girls. The males just flared at each other when they passed by and then continued on their way... Very interesting.
I believe their peacefulness was due to the way they were kept previously in a divided 10 gallon tank. All 3 that escaped had been housed right next to each other in full view of one another for almost 3 weeks and must have come to accept one another's presence to the point that they were no longer violently aggressive towards one another. Two had also come from the same breeder and all 3 were the same size/age. Of course, if I had left them together any longer I'm sure they would have begun fighting over the girls. I'm so glad Apoc didn't manage to escape his or the other escapees would have been in trouble, he's an extremely aggressive guy.
Just fwi, don't spend good money on those betta barracks, they fall down constantly, and are way too small to house bettas for more than a day or two. I have had the same exact thing happen in the past and have lost bettas or had very chewed out fins. It just isn't worth it!
I'll repeat, don't ever house males together!