It's pretty tough to sex Borbs accurately. Some people say you have to look for a yellow mark on the tip of the 3rd dorsal spine, but that's not always reliable. If you really want to pair them off, the most effective way is to get a group of them and see who doesn't beat the snot out of whom. Not pretty, but fairly reliable.
That said, with enough space, even two males seem to tolerate each other pretty well. Mine are in a 240 and even though they take pot shots at one another occasionally, there's no real fighting or behavior I've ever been concerned about. So while they aren't as social as some of their Pseudanthias relatives, you can keep a couple together if you really want to (I'd suggest doing it in a tank at least 100g or greater though).