I generally agree with what Arne said, but have a slightly different take on several things. He is of course welcome to disagree with me if he doesn’t think what I’m saying is correct.
First, in order to give some meaning to the term Spilt Vision other than as simply a synonym for gap, I’ll refer back to Howard Hill, who I believe coined the term, and his discussion of split vision in his book, Hunting the Hard Way. Howard was well aware of the Gap aiming method, and was a straightforward, direct sort of person. I’m sure that if what he was describing was the Gap aiming method, he would have called it Gap. He considered the Gap aiming method to be the method where gaps are mechanically calculated and used to set the gap at known or estimated distances, as discussed by Arne above. I use the capitalized Gap to refer to the aiming method, and the non-capitalized gap to refer to the perceived distance between the aiming point and the point of the arrow, solely to make a distinction between the two.
He used split vision to describe an improvement to instinctive shooting (I’m sure there are those who don’t believe it is an improvement to instinctive shooting, and they are entitled to their opinion). The reason he felt that it was an improvement to instinctive shooting is that if you missed the first shot, you could increase or decrease the gap on the second shot and have a better chance of hitting what you were shooting at. His split vision method does not require that gaps be calculated, simply that they are noticed. When he discusses learning split vision, he does not describe a pathway going first through the Gap aiming method and over time evolving into split vision, anymore than it is necessary to evolve from Gap to instinctive, although if a person happens to first learns Gap and then evolves into split vision, I’m sure that would be fine with Howard too.
I believe (strictly my opinion here) that when Howard made his famous comment about a person needing to decide whether to hunt or shoot targets, he was thinking about the difference between Gap and split vision. If a person is going to shoot targets at known distances, there is no doubt in my mind that the Gap aiming method is superior to split vision. If you know you are at 50 yards, knowing that you need to put the tip of the arrow 24” above the point you want to hit has got to be more accurate than putting the tip of the arrow where “you think it ought to be.” OTOH, if you are drawing down on a deer, putting your arrow point “where you think it ought to be” without even worrying about the distance is likely to be at least as accurate, and certainly a lot less distracting, than estimating the distance and the gap.
For me, Gap shooting is where you are estimating distances and calculating gaps. Split vision is where you are noticing gaps and placing the arrow point where you think it ought to be, based on experience. Instinctive shooting is neither Gap shooting nor split vision, because gaps are not being used to aim the arrow. Just because the gaps are there doesn’t mean that a shooter is using them, anymore than he would be driving in 4 wheel drive in a vehicle if the 4 wheel drive isn’t engaged.