Acabei de escrever uma pequena dica sobre como configurar e executar xsessions separadas em dois monitores. Check it out link
First let me explain what I am going to accomplish to see if this
meets your needs. I am going to be running 2 monitors, 1 is connected
via VGA and the other by DVI. The VGA monitor is to be connected to
screen 0 as monitor 0 on xsession 0, and the DVI being on 1.
Problem that I ran into is that the system was automatically assigning
0 to the DVI monitor therefore not giving me the results that I
desired.
Solution was to simply unplug the DVI monitor, reboot and start with a
clean xorg.conf file. I opened up nvidia-settings checked to see that
my VGA monitor was configured the way I wanted, then I plugged in my
DVI monitor, clicked on “X Server Display Configuration” then “Detect
Displays” and it found the DVI monitor. I then set DVI monitor to be
right of my VGA monitor and clicked on “Configure” and selected
“Separate X Screen” then ensured that all other settings were as I
like. I clicked on “Save X Configuration File” and said yes to merge
the changes. Then I rebooted my system. and viola! I now have my 2
displays running each in a separate xsessions!