[Odd Mathisen]

Vegas does “detect” both cards, but as far as my experience goes it can only utilize one card at a time.

I am running a 6990 which is identified as two separate cards.
I can only select one of them in the GPU acceleration option.
When rendering a video, only one of the cores have a significant jump in temperature(all though the unused core also increase in temp due to sharing the same cooling block as the active one).

There might be ways around this, but it does not work in SLI “out of the box”.

[Odd Mathisen]

I had a similar problem when rendering earlier.
If this is the same issue you’re having(black screen in rendered footage, even though source footage looks ok) do the following:

Click the options drop down at the top in vegas.
Click Preferences.
Under the “General” tab look for “Close media files when not the active application.”
Make sure that selection is not enabled.

Basically what the setting does is clear media file usage as soon as you deselect the Vegas window, which most people do when rendering footage.