Hi,
I would like to know if it is possible to increase the viewport performance with a second card.
Not a big card, but just enough to have a fluide viewport when we have a render region in a 3dview.
viewport performance
if I understand Your question right, then answer is yes =) if You have two cards or let's say discrete card and iGPU /runing from APU/CPU - You could get more fluid navigation. as an alternative tick priority box in "display" or Cuda options.. don't remember where exactly as I don't use that =) that will leave some resources on Your card for monitor =)..
Yes, and the display card doesn't have to be CUDA or expensive.
My Galaxy 210 cost only $10 US, and works great.
My Galaxy 210 cost only $10 US, and works great.

2 GPUs: Zotac GTX 580 1.5gb Amp! GPU for Octane....Galaxy 210 1gb for display
CPU: AMD Phenom II X4 955,3.2 ghz RAM: 8gb OS: Win7 64
CPU: AMD Phenom II X4 955,3.2 ghz RAM: 8gb OS: Win7 64
wow a second graphics card can really make a difference from night to day.
I wasn´t able to use the igpu but Octane and Cycles work now much better with an additional older graphics card which is only active for displaying the OS and so on + I have now more available ram on the render gpu because that one takes now 0 MB on startup.
I wasn´t able to use the igpu but Octane and Cycles work now much better with an additional older graphics card which is only active for displaying the OS and so on + I have now more available ram on the render gpu because that one takes now 0 MB on startup.

Win 7 64 | Geforce GTX570 + GTX260 | i7 3770 | 16GB