If you dont want to invest in seperate aquariums, you can buy dividers to put in larger tanks. Then the filtration etc can be one unit, the fish can see each other which does encourage them to puff up their fins, but they won't harm each other. Putting them together will result in a sad situation. Betta's do not school in the wild; they live in little potholes of water in rice paddies, etc so 1. big tanks make them nervous and 2. they are not social so why force that upon them. At least the divider gives them their own territory that can't be invaded. IMO, the idea of a pet is to put it in a situation that closely mimics how it wants to live in the wild. Sure they can adapt on some aspects but think about how you would feel if the situation was turned. Sometimes its difficult for humans to imagine wanting to live in a small tank alone because we desire just the opposite - lots of free space, socialization etc.
Just my thoughts.