It is never a good idea to keep a male betta with another betta (male or female) undivided. The result would be sickness (from stress/injury), injuries, or at worst death of one/both of the bettas.
When being bred, the bettas have to be carefully monitored and the pair separated not long after the spawn is complete. Even with careful monitoring, death of one of the individuals is not unheard of. Both genders are aggressive and females are just as likely to kill a male as the male to kill a female.