I recently lost a male because the female wouldn't share its food with him.
In the past I have successfully paired them (even a hawaiian with a regular), but usually that's in a big system and then they bond over time. They don't outright kill each other or get territorial but the dominant one doesn't follow the other's actions and doesn't share as well with it, so you'll see them split up often or only one actually eating.
IME; they pair when both are adults, once they realize sex is nice they settle down together, but in the meantime give whole starfish so they both get opposite sides of the food, when one is able to fully cover the star (such as when feeding just a leg) the other sits to the side and waits their turn which never happens.