Keep counting from workbook to workbook

[noelmus]

Hi,
I have 2 workbooks ("Invoice1" and "Invoice2") which are identical
In Workbook "Invoice1" sheets (CHIT1&2,CHIT3&4 and CHIT5&6) have cell J8 updates by double click cell K8 by a code, so if I double click cell K8 in one of the sheets, cell J8 will update in all 3 sheets. So cell J8 in these 3 sheets must always be the same.
Now I need that cell J8 (always in the 3 sheets) in Workbook "Invoice2" will continue counting on that of Workbook "Invoice1". For example if the last value of cell J8 in Workbook "Invoice1" was 000075/19, then cell J8 in Workbook "Invoice2" will start with 000075/19.
This is the code in ThisWorkbook.

Private Sub Workbook_SheetBeforeDoubleClick(ByVal Sh As Object, ByVal Target As Range, Cancel As Boolean)
    Dim c As Range, a
    If Target.Address = "$K$8" Then
        ' Don't edit the cell
        Cancel = True
        ' Increment the invoice nunber
        
        Set c = Sheets("CHIT5&6").Range("J8")
        c.NumberFormat = "@"
        If InStr(c, "-") = 0 Then
            c = "000001-" & Format(Date, "yy")
        Else
            a = Split(c, "-")
            a(0) = Format(a(0) + 1, "00000#")
            If Format(a(1), "00") <> Format(Date, "yy") Then a(1) = Format(Date, "yy")
            c = Join(a, "-")
        End If
        Set c = Sheets("CHIT3&4").Range("J8")
        c.NumberFormat = "@"
        If InStr(c, "-") = 0 Then
            c = "000001-" & Format(Date, "yy")
        Else
            a = Split(c, "-")
            a(0) = Format(a(0) + 1, "00000#")
            If Format(a(1), "00") <> Format(Date, "yy") Then a(1) = Format(Date, "yy")
            c = Join(a, "-")
        End If
        '
        Set c = Sheets("CHIT1&2").Range("J8")
        c.NumberFormat = "@"
        If InStr(c, "-") = 0 Then
            c = "000001-" & Format(Date, "yy")
        Else
            a = Split(c, "-")
            a(0) = Format(a(0) + 1, "00000#")
            If Format(a(1), "00") <> Format(Date, "yy") Then a(1) = Format(Date, "yy")
            c = Join(a, "-")
        End If
    
    End If
End Sub

Thanks in advance