I don't follow what you want but let me give it a try. You want the message
box to pop up if an entry is made into either of the two cells but only if
the other cell is already occupied (not blank). Is that correct?
You first asked to include B1. IOW, if either cell is changed. That code
would be:
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Count > 1 Then Exit Sub
If Target.Address(0, 0) = "A1" Or Target.Address(0,0)="B1" Then _
MsgBox "Don't forget to do this and that also."
End Sub
Also, let me ask you a question. Do you want this message box to appear if
the user changes the content of the cell to blank? Or just if he changes
the content but not to blank? IOW, ignore the change if the changed cell is
now blank?
If you want the message box if either cell is changed, but not to blank, and
only if the other cell is already occupied, then the following will do that:
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Count > 1 Then Exit Sub
If Target = "" Then Exit Sub
If Target.Address(0, 0) = "A1" Or Target.Address(0, 0) = "B1" Then
If Application.CountA(Range("A1:B1")) = 2 Then _
MsgBox "Don't forget to do this and that also."
End If
End Sub
HTH Otto