It will be difficult to take photos as the MP is inside a server cabinet.
But yes I use Pcie riser (the cooler master one) to place on top of the MP the 5700XT (it takes 3 slots room as it is very large) and yes I use an additional PSU (850W Gold) to power the rx580 and the 5700XT.
Pcie1 is rx580 with 2 monitors one 2k and another 4k
Pcie2 is for 5700xt nothing plugged on it
Pcie3 is for the usb 3.2 board (I use an external usb 3.1 24 To raid system)
Pcie4 is for BMD decklink 4k minimonitor
system is on SSD, 2*HD 6TO raid 0 for the cache and 3*HD 8TO raid 0 for the media (all internal)
It is a dedicated editing station, so I sometime also use FCP and Compressor and Motion.
I have monitored that those 3 apps use the 2 GPUs, not al the time, but yes the 2 are used.
I did really many tests, and this is the fastest configuration I found. By the way the rx580 is a real good gpu, I have one in a Razer Core for my macbook Pro as mobile editing station.