StarTech.com USB2DVI - Manual (Page 10)

7
Using the Adapter in Extended or Mirror Mode
By default the USB2DVI will be setup as an extended display. To
change the display as a mirrored/cloned display, check the “Mirror
Display” box on the “Arrangement” page for that monitors Displays
settings window. To return the monitor to an extended display, uncheck
this option.
To use both extended and mirrored displays simultaneously, turn off
mirroring then in the “Arrangement” page press and hold the ‘cmd’ and
‘alt’ keys and drag and drop the display you wish to mirror over the
primary display (shown with the white bar).
Repeat this with each display you wish to mirror, all others will act as
an extended display.
Changing the Primary Display
You can change which display is the primary display by moving the
Menu Bar between displays. Changing the primary display affects
where new windows appear and the position of the Dock.
To move the Menu Bar, click the white menu bar and drag it to the
display you wish to become the primary.