If you are tracking it and really want to make sure you can slide a long way without damaging any of the bike you need both plus the front and rear axle sliders. But the exhaust silencers will still get damaged - there are some type of protection for this - from R&G I think, (but it doesn't look so good). If you have fixed rear set footrests these will also take a bit of a bashing and it's common for the them to bend the frame lugs a bit. If this happens don't worry as they straighten out again without snapping off.