Ultimate member merge fields

Hello,
I have a live search engine that searches on custom fields. To register user i use Ultimate Member. There are fields like first_name and last_name.
When I try search user by field first_name or last_name, then return user, but when i try search by first_name and last_name nothing is happend.
Example user – name: John, last name: Doe. When I search John I find John Doe. When I search Doe, find John Doe. When I search John Doe, then nothing.
How can I merge this two fields into one field.