Avec une macro évènementielle placée dans le module de la feuille :
Private Sub Worksheet_Change(ByVal Target As Range)
Application.EnableEvents = False
If Not Application.Intersect(Target, Range("A1")) Is Nothing Then
Range("A2").Value = Range("A1").Value
ElseIf Not Application.Intersect(Target, Range("A2")) Is Nothing Then
Range("A1").Value = Range("A2").Value
End If
Application.EnableEvents = True
End Sub
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Count > 1 Then Exit Sub
Application.EnableEvents = False
Select Case Target.Address
Case "$A$1"
[A2] = [A1]
Case "$A$2"
[A1] = [A2]
End Select
Application.EnableEvents = True
End Sub