The Find User activity only searches one field and only returns the first value found.  In order to search both first and last name, you would have to combine them in a token first and try to match them against either Account Name or Display Name depending on how your AD is set up.  For example, our Account Name is a user ID so first and last name would not work for that search.

You would also have to watch out for no matches.  For example, John Doe might go by Johnny and using a token would do a search on Doe Johnny and no results would show up.

One alternative you might have would also be building the email address in a token, depending on your emails.  For example, our emails are lastname.firstname@... so I could use the first and last name to "build" them email in a token.  Something like %(RetrieveFields_LastName).%(RetrieveFields_FirstName)@...

This method would have the same issue as AD as far as nicknames being used.  But it might be an alternative to look at.