I used this solution to play rocket league split screen using both my monitors. Now I wanted to play with two friends so I added a third controller. This works fine until the game starts. Then you can still hear sound. From the sound I can hear that the controller input also works. Here are some pictures that might help visualisering what I mean.
I also tried starting a Exhibition match and adding the extra players after starting the match. This has the same result when the third player is added.