Creating a custom split line is really simple, first off you need to create a square texture in image manipulation program of your choice, preferably high resolution (4096 x 4096) since it is going to be scaled to more than size of the screen.

Split line should be drawn vertically through the center of the screen. I’m going with white curve so I can change it’s color through Sprite component. Background texture should split the screen straight through the split line, color the left side for player 1 screen with full red color rgb(255, 0, 0).

Export these 2 textures (line and background) separately and import them into your project. After that, all you need to do is assign them to SplitLine component under SplitScreenUI GameObject.

And you are done, now you will see your custom split line instead of ugly ones that I made.