A buddy of mine did something similar: he has one 300W PS dedicated to the motherboard and the other powers all the other components: hd's, cdrom's, fans...
So it's not exactly redundant but for our purposes might be the ticket... he did have to tap into the mobo control wire to get them both to power up at the same time...
As for the making two into a 600W:
If you are pulling say 400W and one of your PS's go out, then poof... your other one will definately friy as soon as all the 400W is transferred over to it... "kerzittttt!"
Tokamac
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