How to create two dynamic form fields with one space in between?

I have to create a pdf form that works like a letter - one text field for the first name and a second text field for the last name. The text can't overlap each other on really long names or appear with large gaps between the first and last name with a short name. How do I set up the last name field to respond to the ending location of the first name field?

If there is a source that walks through this, I'll take it. I could not find one in the PDF Form user guides.

Thanks!