I have two different text boxes which merge into one, but secondary box can't update

I'm trying to have space to enter information into two text fields ("First" and "Last") which then merges those into one field ("FullName") on a PDF document. I can get the information to load in when I first enter it, but then when I go back to try and change the original boxes the "FullName" box won't update.

Here is the JavaScript I've been using:

event.value = this.getField("First").value + " " + this.getField("Last").value;

Any help is appreciated!