[GiudiceLab]

Our eMagin visor has two VGA connectors on its control box. One is the input from the computer, and the second is an optional output from the box, which they call the 'backfeed'. It's not truly an output, but more like a pass-through or a splitter, since it just sends the received signal back out of the box. We run this output to an external monitor and it allows us to see exactly what the participant sees, without having to set up an additional window or duplicate the monitor display programmatically.

A lot of HMDs have this capability, but if your device doesn't, you could probably use a splitter to accomplish the same thing.