When you find an individual in the 1939 Register you can find other open members of the same household, assuming there are any, by a simple lookup using the items highlighted in the image above. The second one, the location, may not even be necessary. It depends how common the name is.

Copy and paste the name of the additional person and the location into the following two boxes, then click 'Submit".

A link to an FMP page listing all people in a household that has the named person as an "Other household member" will be displayed below here, while at the same time a new browser tab will be opened displaying that FindMyPast page.¹