I don't know anything about bike disc brakes/calipers, but most cars have one or two caliper pistons on a single side, and then the whole caliper slides on pins, thus allowing the brake pads on both sides of the rotor to move toward or away from the brake disc. Only cars with high-end braking systems (Brembo, etc) have pistons on both sides of a fixed caliper.

Are there any bike brake calipers that slide on pins in a similar fashion?