You can place the following code in the exit event of the First Text Field to populate the Second Text Field.
TextField1 Exit event and Java Script..
TextField2.rawValue = TextField1.rawValue..
But every time you change some thing in TextField1, TextField2 will get a new value assigned..If you want only to populate TextField2 when it is empty, then alter the code a little..
TextField1 Exit event and Java Script..
if(TextField2.rawValue != null)
TextField2.rawValue = TextField1.rawValue..
Thanks
Srini