Just the difference I've seen between my double cage and a freind's single, theres a little more grip because there's more pressure on the shoe. Less surface area, more pressure. The second cage supports your foot a bit more and makes it a little flatter on the pedal, making your foot a bit more likely to slip off the pedal. When you have a single, your foot will sag a little into the center of the pedal and give you a little bit for your foot to push against and stop your foot from slipping forward or back. You may find yourself chewing up your shoes a bit more with one cage though... Just my understanding of it...