Private Sub Workbook_SheetSelectionChange(ByVal Sh As Object, ByVal Target As Excel.Range)
    On Error GoTo konets
        If Not IsNumeric(Target.Cells(1, 1).Value) Then Exit Sub
        Application.EnableEvents = False
        [A2].Value = Round(Target.Cells(1, 1).Value / 2.54, 2)
        [B2].Value = Round(Target.Cells(1, 1).Value * 2.54, 2)
konets: Application.EnableEvents = True
End Sub

Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Range("A:B")) Is Nothing Then
    Application.EnableEvents = False
    For Each Cell In Target
        If Cell.Column = 1 Then
            Cell.Offset(, 1) = Cell * 2.54
        Else
            Cell.Offset(, -1) = Cell / 2.54
        End If
    Next
    Application.EnableEvents = True
End If
End Sub

Private Sub Workbook_SheetSelectionChange(ByVal Sh As Object, ByVal Target As Excel.Range)
If ActiveCell = Cells(2, 1) Then
Cells(2, 1) = Cells(2, 2) / 2.5
End If
If ActiveCell = Cells(2, 2) Then
Cells(2, 2) = Cells(2, 1) * 2.5
End If
End Sub