In the InfoPath Designer ....
Double click on the "from" listbox. Click Rules. Click Add. Click Add Action. Choose "Set a Field's Value". Click the icon next to the Field text box. Select the "to" listbox field. Click the fx button next to the Value text box. Click Insert Field or Group. Select the "from" field. Click OK 6 times. Preview.
Of course this assumes that each listbox contains the same values. Otherwise, it won't set. So long as the "to" listbox has the values you are selecting from the "from", it will work.
Patrick Halstead [InfoPath MVP]
InfoPathDev
Patrick Halstead
Project Manager at Qdabra