I think the YouTube video of JohnnyGuru's interview would have been more helpful than the "work it" video.
Curious that your experience changed when you used two separate PCIe power cables though. IIRC, each connection is spec'd to 150W @ 12V. So, even a single cable with two 6+2 would still be 150W each. Considering the 5700 XT draws around 250W at load, I don't suspect your card was starved for power when it black screened/BSOD/etc.