Is it possible to use two GPUs in viewport rendered preview, and not only for final render? I have two Nvidia GPUs and they work fine in normal rendering (when I press F12, both are working).

However, in viewport, when I select "Rendered" as a viewport shading mode, only one is used. Is it possible to use both of them in viewport?
Yes. Open User Preferences and go to the System tab. Under Compute Device, select CUDA and multiple gpus (should be the one with (2x)). Next in the Render context, make sure that Device is set to GPU. It should now use both for rendering in the viewport.