This only happens when using Gnome.
I have an Acer Aspire V3-772GTX54206G1TMakk laptop with an NVIDIA GeForce GTX 760M graphics card running Debian 8 (jessie) 64-bit, and I'm using Gnome 3.14.1.
The problem is every time I plug in a second monitor the brightness of my (built-in) primary monitor falls at 0, turning the backlight off and making the monitor unusable.
Luckily when this happens whilst a session is already open, the brightness can easily be turned back up, making the issue a minor inconvenience. But the real problem is it happens with GDM as well, and when I try to increase the brightness with the built-in keyboard shortcut (Fn + right arrow) the brigntess icon shows up (on both screens even if can only see the second one) but the bar doesn't move. It remains stuck at 0 no matter what I do.
Thus I have found another temporary workaround : Switching to another tty then switching back to the main tty. This somehow resets the brightness to 100%.
I said this bug is gnome specific because both SLiM (as an alternative to GDM) and XFCE (as an alternative to Gnome) work perfectly well with dual monitors. I have tried them on the same computer with the same setup.