Private Sub dropdownlist1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles dropdownlist1.SelectedIndexChanged
If Not Dropdownlist2() Then
Exit Sub
End If
End Sub
Bind dropdownlist2 when dropdownlist1 selected index change
in dropdownlist2 you can add filter of dropdownlist1 so you get only those item which is related to dropdownlist1.