Back when native resolution was 4:3 it made more sense to have them side by side, but 16:9 it just makes more sense to have them vertically stacked.

Also, my setup is a 50" 4k TV for the top screen and a 43" 1080p for the bottom, I'd have to look sooo faaar over to see the other screen that I honestly don't have the wall space in my room for it. I am generally reclined on a couch while using it, my head resting is centered on the upper screen with either a reference of whatever I'm working on the lower, though usually it will be a twitch stream I'm tangentially interested in but only need to take a look at it on occasion if I head something interesting happening.