If you plan on only keeping the 2 of them, it will be 50/50. If your lucky, the larger fish will establish the alpha position and leave it at that. Unlucky, and the alpha will torment the other fish until death. One popular remedy is add more dats. I had an almost 20 pc dat comm w/ all 5 species ranging from 2" to almost 15" with little to no aggression. The alpha was a big indo.
in other words you can almost count on at least a little aggression. GL to you!