Nick Koretsky composed on 2017-05-11 10:20 (UTC+0300):
This is not a mistake, this is a deliberate effort to avoid problems of multiple display in a single screen setup.
Other than using DVI and HDMI rather than dual DVI, and FOSS only,
http://fm.no-ip.com/SS/Xorg/2head-3840x1200x108-nvGT218-big31-deb8TDE-priRight1200.jpg
seems to describe what you are actually after, but I'm not sure I understand what you are after, other than avoiding the screen locking problem that I'm not finding here. I know I'm using one Xorg "screen", and don't think I understand what problem multiple Xorg "screens" would solve for you.