Can multiple video cards benefit to Cycles Preview? I know that multiple video cards can speedup the final render for Cycles. Now I'm just wondering whether it is also true for Cycles Preview mode (i.e. the Rendered display mode in 3D View).

If it is, then how?
Just got chance to receive first-hand answer from Cycles developer:


  Yes, multiple GPUs will also help in 3D View, no extra settings or so needed. It works already.
  
  Actually, in 3D View, it does the same as regular render (by pressing F12)
  if "Progressive Refine" is enabled, which renders all tiles with 1 sample, then again, all tiles with another sample.
  
  One thing should be mentioned is that although GPU got advangtages on speed, a lot of optimizations these days still are CPU only.