Comment 39 for bug 976124

Another thing you could try is assign everything to seat1 and see
if you can get just seat 1 to start:

99-multiseat.rules:

    TAG=="seat", ENV{ID_SEAT}="seat1", TAG+="seat1"
    TAG=="seat", DEVPATH=="/devices/pci0000:00/0000:00:1c.0/0000:02:00.0/*", TAG+="master-of-seat"

lightdm.conf:

    [SeatDefaults]
    user-session=ubuntu
    greeter-session=unity-greeter
    user-authority-in-system-dir=true

    [Seat:1]
    xdg-seat=seat1
    xserver-command=/usr/bin/X -core -config xorg-seat1.conf -layout Seat1 -s 120 -isolateDevice PCI:2:0:0