xfce4-session attempts to manage multiple X screens even though core apps can no longer handle it
Since GTK 3.10, GTK (at least according to the change logs) has not supported multiple X screens (eg. :0.0 + :0.1) from the same process. xfce4-panel no longer supports it, and you cannot start a separate instance because they both will want to grab the same names on DBUS.
Multiple screens are still sometimes unavoidable - for example if you need both XRANDR and the proprietary NVIDIA drivers and have two GPUs that are different.
the GTK people recommend treating the separate screens as different servers, effectively requiring a separate XFCE session for each of the screens (with a separate DBUS). As a result, the code in xfce4-session that iterates over multiple screens leads to the sessions interfering with each other.
It seems that xfce4-session should now only look at the "default" screen, and the loops should be removed.
It would be good if xfce4-session could instead be configured to launch additional sessions (with a new instance of itself and a new DBUS) on the second and subsequent screens. Even better if it could be configured to launch sessions of other desktop environments on the second and subsequent screens.