There is currently not a way to map two Formidable fields to one PDF field, and have your desired result.
To accomplish the result, we would map another PDF field and place it over the current pdf field on the PDF form, that way whichever field is populated in Formidable, it will show on the PDF, and whichever field is conditionally hidden, will be empty.
Let us know if the above works for you, as we remain at your service.