Edit a different page in WP Customizer

The Customizer will apply its changes into the frame over the entire site. Try it yourself with Twenty Thirteen and see. Make some changes, and without saving, navigate to a single post or single page and see how the changes persist even without saving.

So likely you’re doing something else that breaks that functionality in some way. In which case the question isn’t so much how to make it work as it is to find what you’re doing to break it, and stop doing that.

error code: 523