Odds of adding a 2nd tailspot?

[dixiedog]

I have a 29g SPS-dominant tank, with one large tail spot blenny already in residence. I hear these guys can be aggressive to one another unless they're opposite sexes. Is there any way to sex them, or are my odds of failure just 50/50?

 

[zachk]

My male had "streamers" on it's tail fin.