If you are space constrained and want to have landscaping, a 2-lane layout is probably a better way to go. Having to take turns for track time is fine when the people waiting are marshaling the cars for the drivers. The worst thing you can do is to park your kid on a lane while you run around the track doing all the marshaling. When driving time is more precious and the burden of marshaling is shared, driving time becomes more appreciated and skills improve. Likewise, if a kid has to do all his/her own marshaling they may get frustrated and give up. In my opinion, sharing the driving and marshaling responsibilities creates a better balance and it improves the overall experience. It also makes it inherently a shared activity.