There’s a problem when running two X-displays with MythTV – some events on the non-Myth screen will steal focus and then the MythTV controls will no longer respond. This thread describes the problem well, but is now closed for comments.
Since then, mouse-switchscreen has been written, and solves the problem correctly. It’s possible to bind the program to a hotkey.
In the end, I found it better to just run one display at a time since I couldn’t prevent the focus stealing.