Sometimes you just get lucky. I came across this xenia crab on a whip coral at the end of a 70 minute dive and didn't have a lot of time to work on an image. I was so intent on trying to work around toward the left side to isolate the crab against the water background that I didn't even notice the second crab until I reviewed images on a tablet later!
I also failed to notice that one strobe had turned off, and overlooked the not-so-trivial fact that I had inadvertently left the lens set at f4 from an earlier attempt to experiment with bokeh on some coral. Other than all that, the shot turned out OK!
Anyway, aside from correcting the obvious rookie mistakes with lighting and DoF, any suggestions for shooting these subjects in the future would be much appreciated!