How to enable horizontal split in full screen mode in El Capitan
As implemented on iOS 9 and later, OS X 10.11 lets you split the unified full-screen view into two portrait sections with one application on the left side of the partition and a different one on the right. 



It's pretty basic, but it also is elegantly minimal with a clear geometry with only one degree of freedom.

My hope is that as more people get used to the mechanics of this, The software can be complicated to allow us to choose a horizontal petition if we desire to work that way instead of only splitting things vertically. Especially if there are compelling apps that work together in a wide but vertically narrow window.

If you have an excellent use case where you could be more productive at your work, please consider polite activism at http://apple.com/feedback