This doesn't seem to work (at least not anymore). Doing "xfconf-query -v -c xfce4-panel -p /panels/panel-1/output-name -s VGA1" results in the error that output-name "doesn't exist in channel". Doing a -l to get a list of options shows lots of options for panel-1, but output-name is not one of them.
Has this been removed or superseded since? How are you supposed to do this now?
With both monitors enabled, go to the panel preferences and you should get an "Output" option on the Display tab. Select the appropriate output and the xfconf key will be created. I believe the internal monitor names changed in 4.12.