How to make a split-screen? (2 cameras rendering at same time)

Use the Normalized Viewport Rectangle option in your camera. Normalized Viewport Rectangles are specifically for defining a certain portion of the screen that the current camera view will be drawn upon.

It's easy to create a two-player split screen effect using Normalized Viewport Rectangle. After you have created your two cameras, change player one's Ymin value to 0.5, and player two's Ymax: value to 0.5. This will make player one's camera display from halfway up the screen to the top, and player two's camera will start at the bottom and stop halfway up the screen.

Unity Reference link for Camera class here.

Put two cameras in the scene.

In each camera there's a field for "Normalized View Port Rect". These numbers are from 0-1.

So for example if you want two cameras side-by-side you would have one camera at 0, 0, with Width = .5 and Height = 1. The other camera would be at .5, 0, with the same width and height.