I never tried that before, but I just tried it out with a very simple project, including two frames (the mainFrame with a wxCheckListBox and a wxButton that opens the other frame, that contains a bitmap).
The only additional things I had to do (besides what byo said in the post), were to open "wxSmith -> Configure wxSmith for current project" and add the two newly created xrc-files as automatically loaded resources and renew the binding for the button-clicked event to the appropriate function-call cause it got lost.