You know, when I think of it a bit more I think that author designed (as he claims) very fast VNC server as result of kind of lie.
I've read the quote from my first answer once again and I think that the used trick is sort of virtual screen.
If you have to choose screen mode from that other driver and there is a sentence '(you won't see it)'. I think, because that window is open on the virtual screen of VNC server access only.
So, if I'm right, there shouldn't be problem to run two monitor drivers, but this gives you two screens - one real on the monitor, and one virtual for VNC. Now there is choice on wich screen you'll decide to open your window, but this also menas that you can open it only on one of those screens = you can have it EITHER to be displayed on the monitor OR for VNC.
But... this is only, what I guess. :-)