adegner.
Somehow I missed the sli setup you mentioned.  Have you tried rendering to screen with 1 card and then 2 to see if there is a difference in render time?  I wonder if it works like some games where one card will render the top half and the second will doe the bottom half of the image which basically cuts the render time by video card in half because neither card renders the full image but rather a part of it to make it whole.

If you do this test make sure its a render to screen operation in Vue. Curious if you've done this before.