Males are unfortunately too territorial for company most of the time, even other females of the same species. Many people don't seem to realize females are just as territorial as the males, even if they aren't as showy about it, they'll still shred some fins or worse over territory.
Even if a male and female were placed together and they got along for the time being, once they spawned together, the male would drive off the female because of it's instincts to protect the eggs (female bettas are known to eat the eggs, even their own) and one or the other would likely end up dead over the matter. On the same note, if you do stick them together, they will breed (fish will be fish) and then you'll shortly have hundreds of fish fry in your tank too. What would you do about that? They would have to be cared for if you intended upon keeping any of them, and would need tanks of their own quite soon as they grow up and then become territorial themselves. Then again, that's if they manage to grow up, both male and female bettas are known to eat their own fry within a couple days of their hatching. 0_o;
I hear females can be kept together in sorority tanks, which generally consist of at least a 10 gallon tank with 4-6 females in it, no males, as the females will then normally create their own pecking order. But I've no personal experience on that, simply what I've read about online. One must be careful of the individual personalities of the females as well, as some can just be too agressive even with other females - I believe it is for this reason that stores that keep female bettas also house them separately in little cups just like the males, because not all of them can be trusted together in a sorority tank, and it's hardly good for their profit when you start out with a tank full of femael bettas, and the next day have only one or two left.