But why use 2 power supplies? Is there not enough power in the server power supply or is there no adapter to power the motherboard?
The 500-700w ATX unit is only good for 1-2 video cards, but the rig has 8-10 cards, so obviously more power is required. If I had a 10x RTX 3060 rig, it needs 2000w total. The ATX supply will cover 500w, so I need 1500w extra, which can be covered by two HP-DPS750RB supplies. Or I could have an 8-card rig and use a single HP-DPS1200FB supply. I like to have enough wattage to cover the full TDP of the cards.
With 8x RTX 3080 Ti video cards (2800w) a single DPS1200 won't be enough. I will need two of them, with the remaining 400w being covered by the ATX unit.
Why not a 1600w ATX supply or even a 1200w one? Because they are double the price-per-watt of the server units. An EVGA 1600 P2 will cost $220 which is 13.8 cents/W. But a HP DPS-1200FB+breakout board+12v cables costs just $70 --> 5.8 cents/W.
A cheap power supply is not suitable for video cards. It can be used to power the motherboard and hard drive. I do not build a mining farm with a consumption of 2-3 kilowatts. It is better to make 2 mining farms with a server power supply of 1200-1600 watts. Maybe it will be more expensive by $ 100, but it will be more reliable.