I was able to do it! My steps:
- ssh on my recalbox
- cp /etc/X11/xinit/xinitrc ~/.xinitrc
- add this to /recalbox/share/system/.xinitrc
xrandr -s 1920x1080 - uncomment the following line and put my laptop screen :
xrandr --output eDP1 --off - reboot!
I was able to do it! My steps: