I use SPD's. My stoker spouse uses flats.
@
Carbonfiberboy, I'm much stronger than she is, and I have control of the cranks. When I want to coast, I just stop pedaling. I can even backpedal. I guess she detects what I'm doing quickly and gives into it. That's the bright side of it. The dark side is that she doesn't contribute much to propulsion. Riding the tandem is hard for me.