Primeiro digitem o seguinte comando:
xrandr
Terá uma resposta semelhante a esta:
ubuntued@ubuntued:~$ xrandr
Screen 0: minimum 320 x 200, current 2640 x 800, maximum 8192 x 8192
LVDS1 connected 1280x800+0+0 (normal left inverted right x axis y axis) 331mm x 207mm
1280x800 60.0*+
1024x768 60.0
800x600 60.3 56.2
640x480 59.9
VGA1 connected 1360x768+1280+0 (normal left inverted right x axis y axis) 406mm x 229mm
1360x768 60.0*+
1024x768 75.1 75.0 60.0
832x624 74.6
800x600 75.0 60.3
640x480 75.0 60.0
720x400 70.1
DP1 disconnected (normal left inverted right x axis y axis)
Screen 0: minimum 320 x 200, current 2640 x 800, maximum 8192 x 8192
LVDS1 connected 1280x800+0+0 (normal left inverted right x axis y axis) 331mm x 207mm
1280x800 60.0*+
1024x768 60.0
800x600 60.3 56.2
640x480 59.9
VGA1 connected 1360x768+1280+0 (normal left inverted right x axis y axis) 406mm x 229mm
1360x768 60.0*+
1024x768 75.1 75.0 60.0
832x624 74.6
800x600 75.0 60.3
640x480 75.0 60.0
720x400 70.1
DP1 disconnected (normal left inverted right x axis y axis)
Como vêem no meu caso tenho LVSD1 (monitor do portátil) e VGA1 (monitor externo).
Para colocar o monitor do portátil à esquerda e o monitor extra à direita faça:
xrandr --output VGA1 --right-of LVDS1
Para colocar o monitor do portátil à direita e o monitor extra à esquerda faça:
xrandr --output VGA1 --left-of LVDS1
e pronto, ficam com Dual Screen no Lubuntu!
fonte: Dual monitors LXDE (lubuntu) [fixed]