If you have the controllers for other players, there's a relatively easy way to set up a faux-splitscreen game using multiple instances of the game, completely within the ZDoom engine (You may also have to be on Win10.) - me and my BF have been doing this so I can take him through the game.
You basically set up "profiles" (Using the "-config [name]" command in the properties of the shortcuts that you're using to start a co-op game. Though I would set up the configs first before adding the co-op lines.), change their controls (Exclusively to key/mouse and controller respectively - unbind literally everything from the other player's input or it can cause issues. These controls will start very wrong when creating a brand new config file, so I would set these up and test them before you load a co-op game. This is also a good time to pick your characters, as it will remember them per config!) and the location the windows display to (This is more difficult and may require use of console commands and/or a little trial-and-error. If you're using one monitor, going into the config files you made with a text editor - notepad will do - and looking for "win_x" and "win_y" will help a lot here.), then start it as a regular co-op game with each player using their unique "-config [name]" in the shortcuts. If done correctly, you can focus the mouse on the window of the key/mouse player and the controller will still be sending inputs to the other instance of the game. This should be backed up by the controller players' windows highlighted in orange on the taskbar.
Pre-positioning windows for "splitscreen" can be difficult, but needs to be done. This is all because you can not click the controller player's window or it will lose input. I would also recommend opening the key/mouse player's instance of the game last, just to be sure the controller players' instances never become the mouse's focus.
I may write a more in-depth guide for this as I feel it would be a huge deal-maker for a lot of people. If anyone needs help, I'm actually over in the Discord channel for the game!
Tested on Win 7 and Win 10 with one and two monitors! For what it's worth, this was far, far easier to set up than Borderlands 2 was. lol