I would add to Ernie's great advice is that for sports I do not like back button focus because the overwhelming majority of the time you will be using servo AF to follow the action and that is even more critical with wide aperture lenses where only slight subject movement takes the subject out of critical focus. There are plenty of situations where BBF is a great method but I just don't see it for sports shooting.
For example, in baseball the only time I lock focus is when shooting someone at bat and I have my camera bodies set up where the back button is assigned to focus stop instead of activate focus and even in this case I am generally using one of the long white primes that has its own array of focus stop buttons near the end of the lens for this purpose.
So for the overwhelming number of shots I take in sports, using separate buttons to focus and acquire shots would be extra work. In my case, I shoot with two and sometimes three body/lens setups and I will often change bodies during the same play (i.e. 70-200 to body with 400 prime and back to 70-200 body before a play ends). Having to worry about finger placement on two separate buttons when switching cameras would mean losing some key shots because of the slight additional time involved because in sports a few milliseconds makes all the difference between getting and missing the shot.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
