be wary when putting betta's together.
i have 1 male and 1 female living together in a 2ft tank. they get on fine and will breed together. But, if i put the female in with any of my other males, she attacks them and shreds their fins.
every fish has its own personality, and with betta's it will be a case of trial and error to see if they will tolerate each other.
though, if you are getting a couple that have been living together in the same tank previously, then you should be alright