There is a logical issue with what you are trying to do.
As the form opened, both fields are empty. As the user going down the form to fill out the information, he/she has to fill out the first field before even get to the second field (normal user would do that :) ).
As soon as the users tab out of the first field, the 2nd field is empty still since he/she does not get a chance to fill it out.
Now, the script that you put on the "calculate" event will fire every time you exit a field. In this case, as soon as you exit the first field, the calculate event is fire. It will find field 2 is empty and assign whatever value in field 1 to field 2.
So I suggest to check for the value of field 2 to see whether or not it is null on the "exit" event of field 1.
You can try something similar to this on the "exit" event of field 1.
if (InsuredMailingAddrAdvPymt.rawValue == null) then
InsuredMailingAddrAdvPymt.rawValue = InsuredMailingAddr.rawValue
endif