I have used multi screens for a while now but a couple of smaller screens I use are for a GPS panel that I built and the second is a small 10" screen which I use for my main six pack instruments. I have a two PC network and I run P3Dv4.4 on the main pc and the 10" screen along with my standard 19" screen on the client pc. The GPS panel uses a 5" screen with an array of buttons and rotary encoders via a Leo Bodnar USB switch card, whilst the 10" panel is simply connected to the GPs on the client pc.
If you take a look through the Hardware/Cockpit Building thread you might find some more info regarding how I built my GPS panel.
Hope this helps you.