Portal Knights - Vertical split screen is being added to Portal Knights as part of Update v 1.1, coming soon to both PC and console! We know this is a popular request,
8 Best Split Screen games on Steam as of 2023 - Slant
Why play alone? Here are the best split screen PC games! | Eneba
Portal 2 on Steam adds split-screen to co-op mode, Big Picture support - Polygon