I am new user so I may not understand how this function is supposed to work. But, I was able to auto fill the Main sensor with the Select From List button (THANKS FOR INCLUDING THE QHY9). I then checked the Guide Sensor Name box and tried to use the Select From List button to auto fill the Imaging Source DMK21 as a guide camera which only seems to add it as another main sensor camera. I was able to manually fill in the data for the guide cameera and set the offset distance and all seems to show properly when reviewing the selected object in the Field of View Tab.

A valid concern, easily fixed...

Fixed. There's now a separate Select from List button if you want to select the guider sensor only.