How to create a hidden field with concatenation of values from two other fields?

I have a field that captures people's first and last name, and what I would like to do is have a hidden field (admin view only) that concatenates the values of those fields in [last name], [first initial] and then passes it on as an account name. I tried adding the field values (i.e. {Name (Last):1.6}, {Name (First):1.3} ) in the default value of a hidden field, but that just showed "{Name (Last):1.6}, {Name (First):1.3}" in the entries, not "Smith, John". I also tried this without the comma and space.

Other thoughts?

Thanks