Joshua,
IMO given the fact that both the brevis are swimming near the grouping of shells and the one towards the back is smaller and more narrowed bodied, I would have to say it's male and female.
IMO, if it were two males or two females I don't believe they would both be near the shells.
Shellies are very persistant in defending the shells they have claimed as their own. Once they claim a shell, they defend it against all but their mate.
I was able to copy and paste the URL for the pic so I am making my determination from that!!