To create a split screen you need 4 scenes.
Scene one is your game scene with the world and all game objects including the player objects.
The second scene you create is the view. At this scene you need a Node as root element. This root element need two Panels. At this Panels we draw the game. That's the reason why you must organize the position and size as the splits screen should look like.
The next step is to create two scenes. You need this scenes to connect your world with the view. The root node of this two scenes are a Viewport. The Viewport at the scenes has two Children. As the first children you add your game world and second children is a camera.
Now you must write a little script for the camera. At this script you say what object you want look with the camera. Scene one with viewport and camera one should look to player one.Scene two with viewport and camera two should look to player two.
Next you activate the current point at your cameras.
At the last step you go to your view scene and add at the Panel, which should show player one, the scene where the camera is focused at player one.
Now you make the same thing with the Panel which should draw the world around player two. Add to the Panel the scene where the camera is focused at player two.
If I have say clearly what I mean and you have all done correct, you should have a split screen. At the moment it is the easiest way to create split screen that I know.