One question about syntax

Hi:
I have 2 custom columns: #nacionalidad and #nacionalidades. In the first one, I include the demonym of the country of the author; in the second one, I include the demonym of each author in case the book have two or more authors.

To show only the books in #nacionalidad from France, by example, I wrote this in the search bar:

#nacionalidad:"=Francesa"

But, how can I do to include this demonym in #nacionalidades, when in this field normally are 2 or more demonyms? By example, if #nacionalidades = Italiana, Peruana, Francesa, Belga, if I wrote

#nacionalidad:"=Francesa" and #nacionalidades:"=Francesa"

the result do not include any records about #nacionalidades because there are more than 2 data, not only one.

I hope I made myself clear.

Sorry about my grammar.
Regards from Lima, Perú
Blessings