I think I'm missing something...
What I tried to take from your advise resulted in this mess:

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "Periferals!$A$20" Then
Monitors.Range("A20").Value = Periferals.Range("A20").Value
ElseIf Target.Address = "Monitors!$A$20" Then
Periferals.Range("A20").Value = Monitors.Range("A20").Value
End If
End Sub

which I'm putting in "ThisWorkbook" and I'm still not getting anything