You should buy the right stuff if you bought your hashing hardware and you care about how long it lasts on you
if you dont:
I used to do (and still do sometimes) dumb stuff like this too. I think the OP just wants to get more power up and running without having to spend a ton. Especially if you have a surplus of crap power-supplies already.
I work for a small custom computer assembly company, and we have shelves full of useless 300w cheap china crap PSU's that come with the cases we end up not using. they are sitting around wasting space. We use them for various personal purposes and testing and consider them disposable. They blow up, chuck it and grab another one.
check this link:http://www.directron.com/2powersupplies.html
long story short, you can manually power up an ATX powersupply by shorting pins 14 and 15 on the 20 (or 24) pin primary connector.
If you want to rig up two power supplies for your computer, one for each power supply (or more if you are a lunatic) and power them up at the same time it *should* work. Or check the section where it shows you how to build a relay.
I would stick to one crap power supply per juice guzzling video card,
if the powersupplies are very low quality do not run them at the rated load listed on them, stick to 50-60% (if it says 300w on the sticker and its a single 12v rail dont push it past 150)
I have a few Radeon 5770's and 4870's hashing, and I have had experience with running radeons with insufficent power. If the radeon looses juice from the 6-pin connector under a load, (has happened to me) the motherboard should error out and shut down the primary powersupply attached to the motherboard, some of the older ones also have wonderful beepers on them that will scream like a banshee if they don't have enough power. If you are in the building you will know it.
I take no responsibility for whatever crap you break trying to do this. There is a right way of doing things and the "get it done now for $0" way of doing things.
as for the health risks? Try not to lick the power supplies or any exposed wire leads while in your bare feet.