The webmaster of a small home plating website claims to use multiple variable voltage/amperage power supplies, he says he wires them up in a "master and slave configuration", because we need current more than voltage I'm assuming he's somehow wiring multiple supplies in parallel. Opinions on this?

So the power supply has 120 volts going in and two terminals supplying 12 - whatever volt DC. I obviously know how to wire batteries in parallel but how could power supplies be wired in parallel? I mean I cant take the 12 VDC + from the first PS and wire that to the 120v input, what am I missing?

He claims to be using 3 70A power supplies to provide him with 210A.

You can usually parallel the outputs of DC supplies but might have issues with different types. Getting the current shared equally is one. A bummer with high current supplies like you mentioned, is that it takes a ridiculous amount of capacitance to filter the output. This is especially true of a linear supply with 120Hz ripple. The 3 supplies would do well for you if they were fed from three phases and each had full wave outputs. The ripple frequency would increase but the ripple amplitude would be greatly reduced.
Joe

Way back in the olden days when we had massive power supplies in computers they ganged several supplies together with short pieces of nichrome wire that acted as a resistor to balance the outputs.