The transformers in the power supplies provide 'isolation' between the input and output - which means you can connect the outputs together, even when the inputs are on different phases.
Are you planning to build such a machine for your personal home use? If so you should be aware that (a) you might find server hardware hasn't thoroughly tested compatibility with things like suspend; (b) you might find games haven't thoroughly tested compatibility with multi-GPU setups; and (c) you might find the idle power consumption is 200W or more, even while doing nothing.
It's for personal use, though it would not run any games, it would be for running offline inference and other experiments. Probably not a smart purchase, but a fun one...
That is good to know multiple phases can work. Perhaps there would still be a fire risk in case of a short? Like somehow bridging the circuits > breakers don't trip?
Are you planning to build such a machine for your personal home use? If so you should be aware that (a) you might find server hardware hasn't thoroughly tested compatibility with things like suspend; (b) you might find games haven't thoroughly tested compatibility with multi-GPU setups; and (c) you might find the idle power consumption is 200W or more, even while doing nothing.