This is more related to cycling race photography. In another words, your "shot window" is only "open" for 7 to 8 sec before the entire field passes you by. My 1dIII has been my main camera. Recently, I have acquired a 5dII. Curious about full frame and etc. I am still very new in shooting with 2 bodies. I don't even know how to use the second body. Even with one body, I always have enough time to switch my lens. In a typical race, you would get multiple fields racing at the same time with 2 minutes or more intervals in between. So, I always have enough time to switch my lens. I have tired to shoot both bodies at my "shot window." The 5dII with a 70-200L shooting the front of the field. And quickly, I would grab my 1dIII + 24-70L for that close quarter shots. I found myself losing concentrations in the process.