To future Googlers:
Install Nvidia drivers first, and then blacklist nouveau. Reboot the machine.
Your first goal is to make sure that at lease one monitor is working fine with Nvidia driver. In case if you get into graphic issue, press Ctrl+Alt+F1 for terminal access and you can remove/purge the driver OR install a different version of Nvidia driver.
That will ensure both your graphic cards are using Nvidia drivers. If you don't blacklist Nouveau then one graphic card will be using Nvidia and other will be using Nouveau that means you can never get your second monitor working unless both graphic cards use same driver in this case.
lspci -vnn | grep -i VGA -A12 to verify which driver vendor is in use. Check the line "Kernel Driver in Use". If you don't see that line, increase -A12 to -A20 (After the match, number of lines).
Now that you use Nvidia, you can move ~/.config/monitors.xml to ~/.config/monitors.xml.old. Having this might sometime conflict with xorgs. So better move the file and then start editing xorgs.
nvidia-settings command and make resolution auto and enable Xinerama. Save the config to /etc/X11/xorg.conf
Restart lightdm and see if works.
If not, edit
xorgs.conf, tweak everything related to screen 1 and device 1 configuration parameters. Compare it with Device 0/Screen 0 settings and make changes accordingly. Just make sure
PCI/BUS ID is unique and correct one. After proper changes, restart server
lightdm to see if it works. Most of the time the main reason for secondary monitor not working as expected it resolution issue, feel free to test by lowering resolution for screen 1 (secondary monitor) in the config. Either way try and make it similar to working screen in
There are other posts related to this that should help you get it fixed.