You could use a small tank and put a divider in, but if they can see each other it would still stress them out. I don't think any size tank would be big enough otherwise.
i belive betta splendens are 1 per tank (im sure there is a size were they will not be able to come across each other) but they will find each other and fight untill death i did my reasearch on keeping males as they are so pretty but the only way i can see it happening were i can have lots its to either get a split tank or lots of little tanks oh and i also need to persuade the other half on letting me have so many hehe ( ill get there eventually little at a time hehe)
its also very rare that they actually fight to the death, most of the time 1 backs down but still not worth the risk. Glad your not thinking of doing it