Some ideas:
If they are still connected to the hydraulic system on the car, then pull the caliper off of the rotor and insert a smaller thickness block of wood between the piston and press on the brake.
If they are already disconnected. I fabricated an adaptor to allow me to use a bicycle pump to apply air pressure. Starting with a fine thread 10mm bolt, I drilled a hole down the axis of the bolt. I then drilled and threaded an 8 mm stud (also with a hole down its axis) into the top (head) of the 10 mm bolt. The larger bolt should thread into the caliper and the smaller stud should fit most pumps. Again, use a piece of wood that will allow for a few mm's of movement of the piston. Once you get one piston to move out then clamp it to keep it from moving farther out and use a smaller wood shim and try and make the other piston move a few mm (while restraining the first one that already moved). Do not leave too large a gap between the piston face and wood block since the pressure you will need will make the pistons pop out with lots of force once they break their rust bonds. Work each piston a little farther out each time, going back and forth until you can extract them with your fingers. You may need to put one back in and restrain it inorder to get the other one out.
Good luck.
