When i setup Ubuntu, all monitors was detected and works perfectly. After some restarts , things was not working anymore.
I try several commands but this one works
from this topic
The problem is, in my opinion, the lightDM is not great with multi-monitors. There is a good fix though.
sudo apt-get install disper
then edit /etc/lightdm/lightdm.conf to have a
display-setup-script=disper -s line
This will use only the primary monitor for the greeter and unity will take over once your actually logged in.