Concatenate 2 input fields in to 1 field in forms

I input this in the Calculation Properties (copied from an answer from someone's question) but it doesn't quite work.

event.value = this.getField("PatFirstName").valueAsString + " " + this.getField("PatLastName").valueAsString;

This is the issue I am having...  I have 2 forms that I combine with the above calculation set on one of the forms pulling the data from the 2nd form.  Once both forms are combined and saved I have to take the following steps, Prepare Form, Properties, Calculate, Edit, (leaving the script you wrote in the editor box) OK, and close.  Only then will the field populate.  Can anyone help please.