Hello,
Please follow these steps
1) Insert a repeating Section and they insert a Two column table
2) Insert two drop downs in the columns
3) Insert a condition on the drop down 2 to hide this control when the drop down 1 is Blank
4) As long as Drop down 1 is Empty drop down 2 will be hidden
I hope this will help you.
Kalyan G Reddy
Infopath MVP 2007 and 2008