3rd screen seems to be quite common in cab owner and used a lot under VP.
Currently, we have limitations in UP: there is only one render window. We cheat by scaling this render window across all the desktops and adding 2 virtual windows.
Virtual windows (that you see in cabinet mode) are currently in the number of 2: Playfield and Backglass. These are easily extendable and had been though to be limitless and user customizable: in future releases,we planned to give full control about it. The user would then be able to set a dedicated window for the dmd, another for the backglass, and why not a fourth for specific tables like thing in TAF or ransom in BK2K.
But of course, there is MAJOR drawback: performances. If your PC is strong enough, maybe this will be doable. In the meantime, I am talking with hervew to include B2S server in UP: this will dispatch drawings accross multiple cpu/gpu, but, IMO, not as good looking (just my opinion and it largely depends on backglass designs).
So, to be short: it WILL be possible in UP in different manners. But if I where to build a real size cab, I think I'd go for a real DMD, driven by pindmd (now beta supported directly in VPM).
Up to you
. And thanks for supporting UP
.