I saw them do IT

:-)

Two skunk shrimps (Lysmata amboinensis) having a quicky in my tank :-)
Last night, around 1:00am with lights off but ambient light from the
room light I noticed a fresh empty exoskeleton of one shrimp and two
shrimps hanging out next to it... One is niticable softer than the
other - you can tell it by the way how the antennas were moved by
the water currents...
When I was watching suddenly one shrimp grabbed the soft one hug it
belly to belly forming a two-shrimp-cross and hold it so strong that
I was afraid the soft one will crack in half...
Whole thing lasted maybe 2 full seconds and that was it - they
relaxed later with grooming their antennas with their pincers...
Like nothing happened a moment ago :-)

Looks like I was late for the molting show because soon after that
the empty exoskeleton flow with the water current away from the
scene. If I watched the tank couple of minutes earlier I would
have seen a shrimp molting with my own eyes...

Well, I have marine tanks for many years and I had always shrimps
there, but I have never seen before them having sex :-)