Coloca el siguiente código en tu evento Worksheet_Change:
Const col1pos= 1
Const col2pos= 2
Application.EnableEvents = False Select Case Target.Column
Case col1pos
If UCase(Target.Value) = "S" Then
ActiveSheet.Cells(Target.Row, col1pos) = "S"
ActiveSheet.Cells(Target.Row, col2pos) = "N"
Else
ActiveSheet.Cells(Target.Row, col1pos) = "N"
ActiveSheet.Cells(Target.Row, col2pos) = "S"
End If
Case col2pos
If UCase(Target.Value) = "S" Then
ActiveSheet.Cells(Target.Row, col1pos) = "N"
ActiveSheet.Cells(Target.Row, col2pos) = "S"
Else
ActiveSheet.Cells(Target.Row, col1pos) = "S"
ActiveSheet.Cells(Target.Row, col2pos) = "N"
End If
End Select
Application.EnableEvents = True