If you put this code on the code sheet for Sheet1 of your originating workbook it will copy any changes you make to the Sheet1 worksheet to Sheet2 in the same workbook and to Sheet2 in another specified workbook.
Note that the second workbook has to exist. The actual path to the second workbook should replace "c:\ExcelDocs\mytwin.xls" .
Private Sub Worksheet_Change(ByVal Target As Range)
Dim wb As Workbook, bOpen As Boolean
bOpen = False
Worksheets("Sheet2").Range(Target.Address).Value = Target.Value
For Each wb In Workbooks
If wb.Name = "mytwin.xls" Then
wb.Worksheets("Sheet2").Range(Target.Address).Value = Target.Value
bOpen = True
End If
Next wb
If Not bOpen Then 'open the workbook
Workbooks.Open ("C:\ExcelDocs\mytwin.xls")
Set wb = ActiveWorkbook
wb.Worksheets("Sheet2").Range(Target.Address).Value = Target.Value
ThisWorkbook.Activate
End If
End Sub