Rick Welch shoots 3 under and only uses one tied-on string nock, above the arrow nock. Given his success, both in tournaments and hunting, I wonder if this two nock business is overstated. His arrow nocks fit more snugly on the string than is currently recommended, and he tillers his bows (which he makes himself) so the nocking point can be 1/2 - 5/8", the same general location used by those who shoot split-fingers, which are probably reasons why his arrows don't slip down the string. This also makes me wonder if the current recommendations for loose nock fit might also be overstated, and if it might not be more important than is commonly thought to tiller the bow correctly for 3 under.
I personally use two tied-on nocks, and loose fitting arrow nocks, but I wonder if I really should continue doing that. There have been times in speed shooting contests in tournaments that I'm sure I've gotten less points than I could have because of having to place my arrow nock carefully between the two string nocks, and the point made above about possibly nocking in the wrong place in a pressure situation is probably valid. I probably wouldn't do that, but I would look down to nock, so I might miss my shot opportunity altogether.
I think it behooves us to test every bit of common wisdom that we have the time or inclination to do. Much of what we believe right today will be proved wrong tomorrow. Some of what we believe wrong today will be proven right tomorrow. Rick Welch certainly does like to test things, in many other ways than I have mentioned here, which has been a joy to some and a thorn in the side to others.