A quick update on the situation after two years…
I tried again and I was able to achieve this without having to do difficult configurations. So easy for common users as it should be.
What I’ve done: I’am using 346.59 nvidia drivers on Kubuntu 15.04. You make sure you have installed “nvidia-prime” and “nvidia-modprobe” packages. Reboot the system and from BIOS configuration select iGPU as Primary Display and Multi Monitor (or Multi GPU) enabled. On Kubuntu (or Ubuntu) in nvidia-settings in PRIME Profile choosing “Intel - Power Saving mode”. Use Blender with precompiled CUDA Kernel downloaded from official Blender site, or Blender buildbot (tar.bz2 file, unpack to a folder and run “blender” file).
So, Display with the Intel iGPU in the system works fast, while Blender render with CUDA!
Some clarifications:
I have connected my monitor (one monitor) with DVI to nvidia GPU and VGA to intel at the same time. My monitor is set to Auto Detect the input signal source.
If you have had any problems setting this, for go back to the original state rename the hidden folder “/home/YOUR_USER/.nv” and delete files you find on “/etc/X11” with similar names to “xorg.conf.09102015” where the number is the date on which it was created.
Many of the solutions I’ve seen say to just change a BIOS setting, and boom, it works. But my BIOS has no such setting (DELL), and nobody really tries to explain why this works. They just tried it, and it did. But for me, it didn’t, so I dug deeper to try and figure out what exactly Prime was doing (see link).