Certain male/female triggers will pair up. This allows you to keep more than one in a tank.
Cross Hatch, Blue Jaw and Sargassum triggers seem to be the most popular for this. I'm assuming because of their naturally docile temperament.
What do they do? Not much. Just swim around together and maybe spawn down the line, but its always nice to have a pair rather than just a single specimen if your system will allow it.
HTH
Jim