Im putting this in s6 because i dont know if there are any other display managers affected, but basicaly when i have an NVIDIA prime configuration for my 2 gpus in the xorg.conf, and the nvidia kernel modules do not load before lightdm, x starts but is not able to utilize the second gpu.
Ive solved this by appending (and of course running the s6 update db hook) 'modules' to /etc/s6/sv/lightdm-srv/dependencies, so it looks like:
elogind
modules
now.
Thought the lightdm-s6 package should get this update, or at least if not so just to let this thread be searchable and a solution for those who need it.