I know this is not a common setup, but I have a “thick” M.2 card at the back of the Motherboard that is already adding 11mm (Hackintosh stuff to get the WiFi to work). I am guessing the riser cable is going between the graphics card and the motherboard is at least few millimeters thick. That means the space will be too small for me and I might have to swap front and back M.2.
You can shift the GPU riser to the second slot to give you more space. I have done this in my build, so it's definitely possible.