Hello and welcome to the soup,
The number of displays actually does not matter. What matters is the fact you will need two separate windows, one for the salesperson and one for the customer. You simply manage two windows with scripts.
How you display windows on displays depends on the operating system in use. In Mac OS X, both displays share the same coordinate system. One starts at (0,0). The other's origin coordinate depends on the displays layout in relationship to the first, as displayed by the monitors system preference. I assume Windows has a similar mechanism but, alas, I rarely use that OS so can not be certain.