Plug in a second controller and play Skyrim with a friend on one screen, two windows stacked top and bottom.
Play Skyrim Special Edition with a friend on one PC and one screen. Plug in a second controller, press Play, and two Skyrim windows open stacked top and bottom, one controller for each.
How it works
swappads=1 to %LOCALAPPDATA%\SkyrimSplitScreen\split.properties (create the file if it isn't there) and they swap.Optional: the same world (Together mode)
Install Skyrim Together Reborn yourself (version 1.8.4, it needs the Address Library). Then create the file %LOCALAPPDATA%\SkyrimSplitScreen\split.properties containing the line together=1.
After that, Play starts both windows through Together and starts its server for you. Get both windows into the world, then press F10 once, in either window. The mod opens Together's menu in each window, clicks OK on Together's "not purely vanilla" notice (it shows up whenever you have other plugins or Creation Club files), types 127.0.0.1 and connects both players to one world. It takes under a minute and uses your keyboard and mouse while it works, so leave them alone until it is done. Press F2 once to show or hide Together's menu in both windows at once. Add autoconnect=0 to the file to turn the F10 and F2 helpers off; you can still connect by hand with F2 in each window.
Limits
layout=side-by-side in the same file, or if the clicks land in the wrong place on your screen, connect by hand with F2.