I've sent a support note to Larian myself - generally they don't reply, but I presume they get thousands, if not more (!), it can't hurt!
It is an awesome game, and just sooo great that we even have split-screen, indeed many of the additional features/tweaks I personally think are fab. But I agree, the lack of a toggle to go between split-screen and not would be really appreciated.
I would hope that they do occasionally have a ganders at their own forums here. who knows?
A shared-screen toggle really would be useful.
Books are also unreadable in split-screen mode. By permitting a full-screen mode this would make it sooooo much easier to read them (they could also just show a single page of the book at a time, but go figure ;-p )
Pease, the books are just ridiculous in split screen. Lost the motivation to even read them with the minuscule fonts. Also impressive that there is no option to adjust that in the menus. Combine that with all the split screen bugs before patch 3, I think it just shows that the coop was not really tested before launch. Only thing we can do is to keep complaining I guess.