I use xrandr from the command line for this. You dont need to be root.
First run xrandr without arguments just for checking your output devices.
It will look like this:
Screen 0: minimum 320 x 200, current 2624 x 1200, maximum 4096 x 4096
<default> connected 1024x768+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
800x600 60.3 56.2
<VGA-1> connected 1600x1200+1024+0 (normal left inverted right x axis y axis) 367mm x 275mm
1280x1024 75.0 60.0
1024x768 75.1 60.0
800x600 75.0 60.3
640x480 75.0 60.0
I marked the names of the outputs used in this example with "<>". Yours will be different for sure, it depends on the machine.
Now you can try:
xrandr --output <VGA-1> --right-of <default>
Remember to replace the names of the outputs with yours.
If this works, we can check out for an easier way. I think this will be improved in the next release of the Xfce Display Settings app.
Let us know how it works for you.