My brother had male bettas together for a long time. The literature and some other people will say it is impossible, but it is not.
Bettas develop a territorial behavior only if: confined into a small area for two long (like the puddles where they were originally captured from on low tide/dry season...or a tiny cup), reach adult/sexually mature age AND stay in a place that triggers the behavior.
If you got a young male that did not spend a lot of time alone on his cup AND your aquarium is big enough to take away the possibility of him feeling "confined", he will not develop territorial behavior.
OTOH, the risk is, for most, pricey (due to the possibility of expensive fish being injured or killed)
Good luck with your betta.