Comment # 5 on bug 1091828 from Stefan Dirsch

With NVIDIA being primary GPU, both GPUs active and using 'prime-select intel'
after running 

  xrandr --setprovideroutputsource modesetting NVIDIA-0

result is:

# xrandr --listproviders
Providers: number : 2
Provider 0: id: 0x31e; cap: 0x1 (Source Output); crtcs: 2; outputs: 4;
associated providers: 1; name: NVIDIA-0
    output DVI-I-0
    output DVI-I-1
    output DP-0
    output DP-1
Provider 1: id: 0x47; cap: 0xf (Source Output, Sink Output, Source Offload,
Sink Offload); crtcs: 3; outputs: 5; associated providers: 1; name: modesetting
    output VGA-1-1
    output DP-1-1
    output HDMI-1-1
    output DP-1-2
    output HDMI-1-2