Cannot access second GPU on Optimus enabled computer


I have a problem with Optimus. I want to be able to switch between integrated and discrete GPU. On computer with Optimus it’s impossible - from DirectX i have access only to one, selected by Optimus, GPU.

How to get access to the second GPU?

Update: It seems that on Windows 8.1 it is working, but on Windows 10 it’s not working.