Lube the pistons as mentioned, stick one old pad in, find a thin knife/long screwdriver bit and insert all the way through the caliper so you can push against the pad from both ends to give it some welly.
I can push one in no problem, causing the other one to come out further. Then have no way of keeping the first in place whilst fighting the other one back in.
Try slacking off the cable and ensuring the master cylinder is all the way out. There's a valve that needs to open to allow fluid to return to the reservoir. When it's open that path should have lower resistance than pushing out the other piston.
Lube the pistons as mentioned, stick one old pad in, find a thin knife/long screwdriver bit and insert all the way through the caliper so you can push against the pad from both ends to give it some welly.
Flip the old pad and repeat for the other side.