Having used the back button AF method for several years, I thought I would try the dual back button feature made possible by the recent 2.0.3 firmware update. Until the new firmware update, I separated the AF Start function from the Shutter Button using the normal back button method. My focusing method was to have the AF-ON button set for AI-Servo AF. If I wanted to quickly lock the focus, I would simply focus using AI-Servo AF, release my thumb from the button, reframe the subject and shoot. I could have just switched between the two modes by pressing the AF Drive button on the top deck and use the Main Dial to switch between One-Shot AF and AI Servo AF. However, that just wasen't quick enough for my shoot style.
Prior to the new firmware update, you could program one of the programable buttons to toggle between AI-Servo AF and One-Shot AF. However, that required me to press the button set to toggle between the two AF modes (I used the Multi-function 2 button), with my second finger (middle finger) while using my thumb to press the AF-ON button to focus and my index finger (pointing finger) to release the shutter. That's not good either. Sure, I could have also left the shutter button to the default setting (AF and Metering Start-Shutter Release), but I still like separating those functions.
With the new firmware update, you can now program the AF-ON button to AI-Servo and the AE lock button to One-Shot AF. Once set, they are then dedicated to perform those functions when pressed and held in regardless of what is set on the top LCD panel. Having used this now for a few weeks, I find it the perfect match for my style of shooting.
To program these functions, use page 343 of the updated EOS-1D X Manual (downloadable from the Canon USA website). Simply use Custon Function 5 (Operation, Custom Controls) To make the programming changes to the buttons.
BTW, I programed the MF2 button for Spot AF and the DOF button for AE Lock, as I rarely use the DOF button anyway. I also program the M-Fn button next to the shutter button for the Viewfinder electronic level function.
Using the two back buttons for instant AF selection and activation, together with the Shutter button only used for metering and shutter release, add a great new operational capability to the already wonderful camera. I just wish I could setup my 1D4 to do the same!
I realize this was a rather lengthy explanation, but I know I like it spelled out fully when someone suggests a new method to me! Hope this helps! It is definitely worth trying!