I also thought it had been determined that Vegas only uses the GPU on one card anyway.

Not that i would know anything about this stuff. I still don't have even one GPU in my editing machine.

When I tested this about two years back I tested a GTX570 with HD6970. In my case, both cards fall within the fully supported cards range. The other major difference is that the 5820 CPU has only 28 PCIe lanes versus 40 on the 3930K and that the OP mixed PCIe 3.0 with 2.0.

In my setup, I had the HD6970 in the lower slot and the GTX570 in the first slot. It is also important to use the right drivers, in my case that was Nvidia 296.10 and Catalyst 13.7. The HD6970 was set in Vegas for GPU support but I used CUDA when rendering with MC AVC. This was the fasted rendering times I ever got. The HD6970 was preparing the frame with OpenCL and the GTX570 used CUDA to render it out... so to speak. Not even SCS could explain why this worked but it did.

If I find some time I will do some more testing as I still have one of my GTX580's lying around. Unfortunately, the water blocks on the Nvidia and AMD cards have there water connection in different locations making it difficult to have these two cards in the system; I need to buy some fittings to get that done but I am still interested to see how that will workout.

Thanks Oldsmoke. The weird thing is that it works a day or two, and only then starts to crash.
I'm also not really after shorter render times, I mainly want a smoother preview.

A weird thing I also strated to notice is that when putting clips on a video track above another video track, preview is slowed down although Vegas only has to play the above track that covers the others. It's as if Vegas read and processed the track underneath alhtough it doesn't have to.

@Mindmatter

If it worked for a day or two and then doesn't that sounds more like an automatic Windows update upsetting the system.

The issue with Vegas slowing down with another track underneath has been discussed in this forum. I personally have not experienced it but keep in mind that Vegas is a compositor too and if there is even the slightest chance that something from the track below "shines through" the track above, Vegas will process it.

Oldsmoke, I did check the drivers mutiple times, and my auto update is disabled in Windows. It's quite weird indeed.
I started to mute video events underneath the main track and it does seem to help.
I also noticed that Titler pro did not quite seem to be able to decide which monitor to consider as primary. I'd work it on my secondary screen and as soon as I minimized TP, it retired to the other one, when enlarging it again, it would show up in a reduced form on the main monitor and I had to drag to and maximize it again on my secondary screen.