Well HashFast didn't design the system for redundant power supplies (it would require much higher output power supplies = higher cost) but pulling double load from one power supply won't necessary "bad things". It would cost more but if each power supply had sufficient capacity to handle the full load then the system could operate just fine on one. When both PSU are operating the load on each one will be half. If one shuts down the other will handle the full load. You gain redundancy, and higher efficiency at the expense of more power supply cost. If one PSU fails the other will run at 100% load until the first PSU is replaced.
Each board pulls about 300 watts over 2 PCIe 6-pin connectors. When one power supply dies, that board will start pulling 300 watts over a single 6-pin connector. How many power supplies cables can safely supply 300 continuous watts over a single 6-pin connector?
Just about all decent brands (and certainly SeaSonic). The 6 pin = 75W, 8 pin = 150W is a PCIe standard, it doesn't represent the limit of what is possible using the underlying hardware. The connector itself can handle 288W continually, Molex rated specs for 6 pin Minifit Jr connector is 9A per pin (13A with high current pin). So 6 or 8 pin PCIe connector is fine for 3 conductors *12 Volts * 9 A ea = 288W and real world is probably significantly more. Don't do this at home but I stress tested a connector at 420W for over an hour and it was only warm to the touch. The wiring itself (3ft, 16 gauge, 12V, 20A per conductor) can handle a lot more so it isn't the bottleneck.
The only difference between a 6 pin and 8 pin connector is the 8 pin has a pair of ground sense pins to compliant devices (i.e. GPUs) to know which cable is connected on power up. If you look carefully at many power supply cables you will see there are TWO 8 pin connectors in series on the same cable which plugs into the PSU. Even under normal usage that means 150W + 150W = 300W on the wires.