+1 for separate. Today's Bettas are as unpredictable as they ever have been. ( Before, they were 100% predictable. You definitely could not keep them together. ) For every success keeping multiple males or multiple females or even males and females together, there are failures to match them. The fish was designed for fighting so you should go with that premise when purchasing them. The fighting fish have been redesigned to have different fins so you will not know how much fight is left in the fish until you see it. You are just playing the odds when you try it.
Hope this helps