The important thing is to edit the X and Y offset on the second Xibo
instance, these X and Y offset will depend on the display resolution of the
first display.
I have a documentation that I did myself for this, pls see attached.
You will want to specify player size as well, then assign that second display profile to your second player, you might need to restart the player for it to pick up new profile, after that it should be fine.
You will most likely want to adjust watchdog configuration files for the second player (so it can monitor the second player correctly), perhaps have a look at the more detailed notes here.
Manual will be updated with those in the next release.