Larian has delayed the release of Baldur’s Gate 3, currently on pace to possibly be 2023’s Game of the Year, until they can figure out how to make split-screen work on Series S.
I’m not sure. I saw some one comment that they were running split screen on Windows, but I can’t personally verify that. Based on what I know of software development, it’s likely part of every version of it but not necessarily easily accessible. For example DoS2 has split screen coop on PC, but you wouldn’t know it by looking at it. You have to plug in 2 controllers and do some extra steps but it works.
Maybe if you plug in a second controller on steam deck you can?
EDIT: missed an ly on likely
No. You can run split screen on non-Steam Deck PCs, and in fact you can launch BG3 on a Steam Deck as if it were a proper PC with split screen enabled (it prob just won’t run well).