I made it work. Shat I did is place an action on the check box properties and selected a trigger to run a Java Script.
Here is the code.
//Set the vars one and two:
var one = this.getField("BillingAdd"); //Text Box 1
var two = this.getField("ShippingAdd"); //Text Box 2
var cb1 = this.getField("ShipSameBill"); // Check Box
//next check if two is blank and if so, populate it with one’s value
if(cb1.value=='Yes'||two.value==null){two.value=one.value}
else {two.value=''}