Formula help!!

Hello!  I have been working with Sugar for 3 years, but have never had much experience with calculating formulas!

I have a field called Closed by. 

I want this field to populate with the name of the Assigned to user once the case Status has been changed to Closed.

I have tried everything I can think of and cannot get anywhere.  Also, when I use the Related to formula to get the Assigned to User name to populate, I can only choose first name or last name and I need to be able to see both.

Any help you can offer is greatly appreciated!!