They will most likely have a bit of a fight, tear some fins maybe batter a couple of scales and then the loser will back off and the winner will just swim around lording it over him.
All bettas are aggressive. It's just the varying degrees of aggression between the different species and complexes. Most aggressive bettas I have are Betta ideii. It has been known for partners to kill each other or for females to harass and kill their males while they are holding eggs.
My female and male were intent on killing each other when I first got them. Then my female recently tried to kill my unimaculata male when I tried him in with her.