2 Listboxed populating FlexGrid [Resolved]

I have 2 Listboxes, a datagrid with 2 colums and an add button on a form.
I want it when a item on the 1st listbox(source) is clicked and the add button is pressed, it gets added to the first column in the flexgrid. When an item from the second listbox(destination) is clicked and the add button is pressed it will add the item to the second column in the flexgrid.

I have that working alright, but i want it so that only 1 listbox can be clicked. Right now it is possible to have 1 item from the 1st listbox selected while one item from the second listbox is also selected. I want it so that only 1 listbox can have items click on it at 1 time, but be able to switch back and forth.

Heres the code i have so far.

VB Code:

Private Sub cmdAdd_Click()
 
 
If lstDest.ListIndex <> -1 Then
lstSource.ListIndex = -1
   Item = lstDest.Text
   If grdFields.TextMatrix(0, 1) = "" Then
      dGridRow = 0
      grdFields.TextMatrix(dGridRow, 1) = Item
   Else
      dGridRow = dGridRow + 1
      grdFields.TextMatrix(dGridRow, 1) = Item
   End If
End If
 
 
If lstSource.ListIndex <> -1 Then
lstDest.ListIndex = -1
    Item = lstSource.Text
    If grdFields.TextMatrix(0, 0) = "" Then
       sGridRow = 0
       grdFields.TextMatrix(sGridRow, 0) = Item
    Else
       sGridRow = sGridRow + 1
       grdFields.TextMatrix(sGridRow, 0) = Item
    End If
End If
End Sub

-Ryan

VB Code:

Private Sub lstSource_Click()
    If lstSource.ListIndex >= 0 Then
        lstDest.ListIndex = -1
    End If
End Sub
 
Private Sub lstDest_Click()
    If lstDest.ListIndex >= 0 Then
        lstSource.ListIndex = -1
    End If
End Sub

Thanks,
Worked Perfect, a lot easier to do than i though.