I might be saying something foolish now, but it seems to me that you are trying to get the Amiga to generate two different screenmodes at the same time.
But I don't mean in the same sense as I may have two programs running simultaneously, one at 31kHz and the other at 15kHz, and I can drag the front screen to see the other program running in the back screen, adopting the screenmode of the program in front, on my 1438s multisynch.
No. What I mean is a genuine two-monitor display, in other words running one program through the 15kHz video port and the other through the 31kHz VooDoo. It seems to me that if this was possible you would be able to watch a 15kHz native demo on your TV, while raytracing with Cinema 4D on your PC monitor, with changes to the one screenmode not affecting the other.
I don't think this is possible. What I mean is that the Amiga is choosing what to display, and it either displays it through the 15kHz video port, OR it displays it through your Voodoo.
Am I right or have I lost the plot here?
edit: chose the wrong name for a 31kHz-friendly trace prog. Fixed it.