I do run into those situations of course, where I want to see two windows side-by-side, but it's very rare and almost always temporary. Most of my windows run full screen, and if I need to focus on something else I just switch to it. It's important that this switch happens very quickly, so my window manager configuration has no delays, and every window is a quick keyboard shortcut away (either on a different workspace, "alt-tabbed" on the same workspace, or quickly accessible via rofi).
This workflow completely removes the need for a separate screen. A larger screen and higher resolution would be nice to have, but not for productivity reasons. I've found a 15.6" 1080p display to be sufficient with this setup, even though I have other screens I could use at the same time. It boils down to personal preference, but I doubt there's some "empirical evidence" that using more than one screen objectively leads to higher productivity.
This workflow completely removes the need for a separate screen. A larger screen and higher resolution would be nice to have, but not for productivity reasons. I've found a 15.6" 1080p display to be sufficient with this setup, even though I have other screens I could use at the same time. It boils down to personal preference, but I doubt there's some "empirical evidence" that using more than one screen objectively leads to higher productivity.