Add aListing that belongs in Multiple Categories

Hi

I am wondering how I add a listing – using the “Add a Listing form on the site” in a CPT that belongs in more than one categories (within the CPT). The “default” shows one category field.

I don’t see how I can add another category field to the CPT as this would violate the rule of uniqueness.

Hope this is not another stupid question.

Thanks
Stacey