Yeah it would need to be a pretty powerful spring.
I thought of where I would put the spring - it can go around the stationary shaft, with one end attached to the clutch of the top cymbal, and the other end attached to the clutch of the bottom cymbal (so it's coiling around the part of the shaft inbetween the two cymbals). That will keep the cymbals tightly clamped together until you step on the pedal.
I'm trying to come up with a way to retain the top cymbal as the one that moves and still have it open up when you step on the pedal.....