I created a split screen before in 3D.
All I did was create two viewports, each with their own camera, and show their textures in two texture_rects.
The same should apply in 2D, except you would need to use a camera2D instead (duh). And you may need to clear the world2D that it comes with.
view_port.world_2D = null
By the way I was basing that statement off of the viewport document, which looks kinda outdated; so you may want test that out yourself.