you can set a Field Event trigger (Document properties>Tirggers) "On select" for the field of the second list box. The action wil be "Clear Field" in which you will set the field of the first list box.
Let me know if it's not clear!
thanks rgv rand in fact I checked but the problema here is that , when I tried to configure some condition in the part of 'ON SELECT' it not Works , I mean if I put only the field that most be cleaned it Works but when I put some 'IF' statement then it doesn't work.
and why and 'IF' statement, because only want to clean the list box when some value be selected ,I mean not in all selections of the same field.