When you see people online showing off their computer setups with dual displays, you often see side-by-side horizontal monitors (landscape mode). Sometimes you see a horizontal screen and a vertical one (portrait mode). And sometimes you see stacked displays, with one landscape-oriented monitor mounted on top of another.
Sometimes you see the stack because of space issues, where there’s simply no room to either side of the setup. Other times you see a stack when someone wants to run four or five displays. And there are cases where the user couldn’t get one monitor to work in portrait mode, so they had to have both screens in landscape mode.