I used a rubber stopper drilled out in the center to plug the missing cylinder. This allowed me to plug the missing cylinder and have the other stuck piston pop out.

I adapted a diesel injector tester pump to a caliper brake line. Pumped them full of water to remove the pistons. I Originally used brake fluid but it made a huge mess.

Dang, Water does not compress, Super clean and easy.

I think I used a 1.5" rubber stopper. Worked awesome.