Need hidden field populated with values from multiple fields

I have a hidden field (NAME). For integration reasons I need the client to fill out first name and last name separately, and I want those values to automatically fill out the hidden full name field (which I need for a separate integration). I have tried using conditional logic, but the full name field is still blank.

First Name: John
Last Name: Smith
Automatically fills in hidden field Name: John Smith

I tried it and unfortunately, it's not possible whether hidden or not. A workaround would be to populate First and Last Name into another text box field instead of the Full Name field. 

Just tried, created a general field, added conditional logic to fill from first last, still coming up blank.
I desperately need this to work with integrations for Highrise and Dropbox.

Noticed that for the conditional logic I'm choosing "Copy Multiple Field Values", clicking save but it is changing that to "Calculating Field's Value" every single time.  Maybe that is the issue?  Bug maybe?

That is because you're using the target field for the IF rule. I've changed it for you, IF first and last name fields are filled:

Rookie mistake.  Thanks for the fix!