Two so called 800 watt power supplies can only supply half their rated power. I built a brand new Ethereum mining rig using 5x RX 580 graphics cards and two 800 watt power supplies.
The mother board is the Biostar TB250-BTC which has special bios settings for mining and six PCIe slots. I sometimes have trouble getting the 6th graphics card to work on a mining rig so I only got 5 cards for this rig.
I first got Windows 10 pro installed on the SSD hard drive and only the motherboard, CPU and RAM. I am using a cheap LGA1151 Celeron processor and a single 4GB DDR4 RAM stick.
After getting the on board graphics card drivers installed and working I put on the first RX580 graphics card and mined Ether for a while to make sure it was going to be stable.
After that I added the second graphics card. Now I was only using a single 800 watt power supply at this time. The motherboard/CPU are using about 100 - 120 watts, including the hard drive.
Out of the box each RX580 is using about 220 watts of power. This should be just over 500 watts of power on average. But the so called 800 watt power supply could not handle the power of the motherboard and 2x RX 580s.
I added the second power supply and continued to ad on graphics cards. The second power supply could only handle two RX 580 graphics cards at a time before it failed on me. That is only half its rated wattage but it could not take a third card.
I had to get out one of my Thermaltake power supplies to supplement the power needed to run this rig.
The cheap power supplies were not that cheap. They were actually more expensive than an equivalent Thermaltake power supply. I had figured for this rig I would buy a more expensive power supply to be safe.
From now on I am sticking with name brand. Thermaltake is the brand I trust now.