Suspect there are as many answers as there are pedals (or at least half of them, assuming even pairs throughout) however will offer my £0.02.
There are two things I want from a recumbent clipless pedal:
1 - Certainty of engagement.
2 - Float.
1 is the thing that stops me kicking a leg skywards when I think its engaged but it isn't; that keeps me upright in those precious few strokes setting off; that matters more when I don't have the weight of my body pushing *down* on the pedal to keep contact, even if I'm of the plate.
2 is the thing that stops my knees from hurting.
A single sided pedal is compromised on 1, and I'm yet to find a pair of shimanos that offer enough float. Personally, I'm a fan of Time Atacs. Cheap, light, sturdy, solid, floaty, great.