Well, i have downloaded version 8.0 from this website :
https://archive.org/download/thc-recalbox
I have modify the cmdline.txt with this : video=HDMI-A-1:D video=HDMI-A-2:D (source : https://forum.recalbox.com/topic/26323/recalbox-8-x-sur-pi3)
And with this version 8.0, it works !!!
When wifi was set up, i have selected update from the update menu.
It has downloaded version 8.1.1 and installed it offline ... and then ... nothing ...
Seems like if this version 8.1.1 doesn't work 😞