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.
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).