BG3's PC minimum specs list 4gb vram and 8gb normal ram. Assuming windows uses 3 gb, that's 9gbs of total memory that the game needs. They could just use lower res textures when in splitscreen and be done with it, but I guess they want to compromise as little as possible

Edit: apparently Microsoft wants games to use less than 6 just in case someone tries to activate all background functions at once. That is indeed quite stupid.