Private Sub ComboBox2_Change()
If ComboBox1 = ComboBox2 Then
MsgBox "You selected this number in ComboBox1. Select a different
number here."
ComboBox2.ListIndex = -1
Else
If ComboBox2.ListIndex -1 Then MsgBox "Supply msg."
End If
End Sub

Maybe you want similar code behind ComboBox1, since a user could
select the same number in both by selecting from ComboBox2 first.

Hth,
Merjet