This is on Arch Linux. After an update, my 2nd monitor was no longer detected. I downgraded nouveau to the Arch package version 1.0.15-2 (from 1.0.15-3) and the problem went away. I also had to downgrade xorg-server due to dependency issues, so it's possible the problem could be there and not in nouveau itself. My xorg-server package is now xorg-server-1.19.6+13+gd0d1a694f-2, down from xorg-server-1.20.0-2. Both monitors work fine in the downgraded configuration, but only one works with the latest updates. When things weren't working, /var/log/Xorg.0.log showed the message "failed to find screen to remove" for one of the GPUs. This is no longer the case after the downgrade.
This might be the same problem: https://bugs.freedesktop.org/show_bug.cgi?id=106586
It might be the same problem, yes. It definitely looks similar.
-- GitLab Migration Automatic Message -- This bug has been migrated to freedesktop.org's GitLab instance and has been closed from further activity. You can subscribe and participate further through the new bug through this link to our GitLab instance: https://gitlab.freedesktop.org/xorg/driver/xf86-video-nouveau/issues/434.
Use of freedesktop.org services, including Bugzilla, is subject to our Code of Conduct. How we collect and use information is described in our Privacy Policy.