I came across posts suggesting powering the riser and PCI-E power connector on the card with the same PSU. Others say the psu that powers the motherboard must also power alll the risers. The slave psu should only power the PCI-E power connector on the cards.
https://bitcointalksearch.org/topic/m.17825999
https://bitcointalksearch.org/topic/m.18351224
Ultimately, I did follow the latter suggestion as it seems logical the same power supply that powers the motherboard should also power the risers connected to the PCI-E slots on the motherboard. The second slave power supply only powers the 6 & 8 pin auxiliary power connector on the video cards. According to another post I came across that connecting the PSU's that way ensures they share the same ground, which is important.
https://bitcointalksearch.org/topic/m.4878774
I also noticed BBT in some of his his latest live streams emphatically recommended that when setting up a dual PSU rig, you need to power the risers with the same PSU that powers the motherboard and only use the second PSU to power the GPU's.
https://www.youtube.com/watch?v=lXZondkAWVc&feature=youtu.be&t=1253
https://youtu.be/pQ-EAunoAqY?t=988
I have a EVGA 850W G2 and 750W P2 setup tpgether with an add2PSU relay. The 750W powers the motherboard, peripherals and all of the risers. The 850 W only powers all of the 6/8 pin VGA power inputs on the cards.