I'm assuming these two weren't fighting as there didn't seem to be any struggle, and I didn't want to grab them in case they were otherwise engaged, so . . . anyone know if kelp crabs are into the missionary position?

Belated update: Minette says,

Often I find crabs like this...with two snuggled up together and an apparent third off to one side. The third invariably turns out to be the carapace of the female who has to molt immediately before mating so that her body is soft enough to accept the male's sperm packets. The male will embrace/protect the female like that after mating until her new shell hardens.