FindControl only searches for children immediately inside of the specified parent control, which is why you are having the problem.
Since both RadComboBox controls are in the same edit template, they should have the same Parent, so an easy way I can see to do it would be something like this: (Pardon me, my VB is rusty)
RadComboBox = TryCast(o, RadComboBox)
RadComboBox = TryCast(sourceDDL.Parent.FindControl(
Hope that helps,