To run two, simply go to the hardware store and get a crimp-on splice connector then cut out the partiton on one of them (obvious when you see
IT in person.
You simply clamp that onto both PSU's Power-On (Pin 14, usually green wire) leads and make sure they share a common ground (screwing both PSU's metal casing to the system case may suffice but otherwise choose how you want to do it, there are several ways).
Doing this results in the motherboard turning on and off both PSU together.
As already mentioned, it's better to use one good PSU than two junky ones. Inevitably one PSU bears a larger load per rail, and needs be fairly beefy. You could use a lesser wattage (but not junk!) PSU to power other things, drives or video card, etc. There is a definite advantage to using two GOOD PSU instead of one very good. Take two $35 (on sale at newegg) Thermaltake 420W for exmaple. Together they have an ACCURATE 12V rating of 36A. That's more realized output than when somebody like Enermax claims 36A on their label, PLUS the heat density per each PSU is (not halved but a lot lower still), in addition to their being doubled fan exhaust rate. For 5V amperage, there IS NO standard PS2 form-factor ATX PSU AT ANY PRICE that can output more 5V amps than a pair of aforementioned $35 Thermaltakes (or take your pick two even better PSU aren't necessary but would be more likely to have enough power for peltiers, etc).
So there are a number of benefits, but bang for buck is real high up there too. PS2 sized PSU simply weren't designed large enough for long-term operation at over 350W true output- we're still stuck with designs set when avg. system used < 80W.