Yes, definitely keep a divider between males! they will fight until one is dead, no matter how big the tank, since the "loser" does not have room to run away as in the wild. and make sure your divider has no gaps on the bottom, sides, or top (it needs to butt up against the underside of the plexiglass that is under your light - install a top barrier of this sort if you don't have one already, since they could jump the divider). you will notice the boys trying to "tear down" the divider to get to each other, at least for the first few days. would be best to get a divider made of plexiglass, plastic, or glass, with lots of water flow slits in it since the mesh style dividers pose a hazard in the male betta trying to bite through it and possibly hurting or snagging his mouth/teeth. some people have said that a color tinted divider works best since it doesn't allow the fish a super clear view of his neighbor and keeps them from getting too stressed until they get used to the new "boy next door" (flaring continuously is very stressful for males).
Also to note that you may want to put a sponge filter (thats what i use) on each side of the divider. if you have only one filter, the fish on the same side of divider as the filter will get better water quality than the fish on the non filtered side. one heater should be enough as long as you put it as close to the divider as possible.
Same as briggadane, i keep two female bettas of different species (one veiltail and one crowntail) in a 10gal together, with no issues yet after a week, in fact they sleep near each other (keeping fingers crossed - had to remove the third female yesterday, they were both picking on her). Like all are saying, be on standby with the net! .... ever yell at your fish? "stop that! leave her alone!" lmao
My males are housed side by side in seperate tanks right now, so when i go to divide my 20 gal for them i hope to not have too much of a time getting them to settle down.
Feel free to PM me for any info needed on bettas. I'll admit I am far from being an expert, but the info i've gathered thus far is enough to keep all my fish happy and healthy (knock on wood) since the day i got the first one and now i'm up to six in less than six months, with hopes to begin breeding soon.