The main challenge for this change appeared to be lightdm:
it can do dualseat but in a quite peculiar way tied to systemd;
while wdm-based implementation well prepared and described by viy@
can survive both systemd and sysvinit just fine.

branding-alt-workstation-mate-settings pulls in lightdm-gtk-greeter,
and lightdm takes over; it should probably be masked in systemd case
(sysvinit should need no further hassle as /etc/X11/prefdm considers
wdm before lightdm for historical reasons; or /etc/sysconfig/desktop
could be filled in to be sure) *but* the current stopgap "solution"
is what it is now.

See-also: http://altlinux.org/X11/DualSeat