Usually if one piston is stuck, then the caliper piston is hanging. If both are stuck, the master cylinder is not releasing pressure.

Have you tried to open the bleed fitting on the rear caliper to release the pads? If that will allow the pad to retract then there is a problem in the master cylinder not releasing the pressure. Be sure that some debris hasn't gotten down into the small bleed port in the floor of the master cylinder reservoir. Also, highly unlikely but make sure that the reservoir is not overfilled when cold.

If opening the bleed fitting does not release the pads, then the cylinders are sticking.
The caliper mount bolts go through sleeves that need to be lightly greased to all a little movement when the caliper is activated and released. Be sure that the caliper isn't hanging on the sleeves.