You don't want to mix-n-match cables. If you have 2 power supplies feeding to a single board and one power supply dies, the board will try to pull double the load from the other power supply. Bad things happen.
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.