try to wiggle a razor blade in between them, then a flatblade screwdriver. you have to manually push them apart. be careful because this will likely cause some damage to the pads. but you can lightly sand them afterwards.
You should still be able to get a flat head screw driver in between the backing plates (since the pad friction material doesn't go all the way to the edge) and "twist" the screw driver to start the pistons back into the calipers, that should create enough of a gap to then get something between the pads to push them back as needed.