I just wonder if we could compromise by letting Petr play in both domestic cups and letting Courtois loose on the PL and CL!
Best thing you can do is rotate them around. You'll be playing PL, CL and cup games! Both will have time to play. I'd personally select Curtois for the PL games though. He's young and needs to develop more. Cech can make do with CL/Cup games for now.