I can’t tell you how many times I’ve read about the benefits of a split yoke on a shirt. And by benefits I really mean two things. The first was that a split yoke was the only way to properly match the pattern of a shirt between the sleeves and rear part of the shoulders/upper back, creating a much cleaner aesthetic. The second of which was that it often meant a higher quality shirt, as split yoke construction took more time and skill than a simple one piece yoke to produce. It was not until I read a piece by Arnaud Rousseau (who is an upstart shirt maker, currently he has only RTW shirts but MTM is on the way) that I heard of the functional advantage of a split yoke. I recommend you give it a read, it is quite interesting and informative. But in short, the advantage lies in how the shirt fabric stretches more when pulled at a diagonal (which is the case with a split yoke), not straight along the warp or weft, as would be the case with a single yoke. This allows the shirt to stretch a little further when one extends their arms, causing less tightness in the back and as a result more comfort.
You can test this theory on your own with any of your dress shirts. Pinch the fabric of one of your dress shirts with your fingers a few inches from each other and pull parallel to the weave of the fabric, you will notice the fabric does not stretch at all; as you are pulling directly on the cotton which itself will not stretch. Now, pull at an angle to the weave, you will notice how the fabric stretches. Case closed.