I have no clue if they are M and F. I got the smaller one about 6 mths after i had the big one and there never seemed to be an issue. They dont hang out together, they both just swim thru the the whole tank at their own will. When they bump into each other they just seem to ignore the other and just keep on swimming. They have never seemed to act as if they were going to mate or anything and I have never reaserched how they mate, but I will now that I think about it.
The larger one does often face off to its reflection as ophiura mentioned, but never any attempt to attack the reflection as far as I can tell.
Now the yellow tangs...those guys do try to attack their relections, but not the other Yellows in the tank....strange....this hobby we enjoy.