Is it possible to hook it 2 computer power supplies together? For ex, a 200 watt psu hooked up to a 300 watt psu and the total wattage output would be 500 watt.

No it doesn't work that way. You could run 2 power supplies seperately with some devices connected to one and the rest to the other. You would then have a total of 500 watts available but split between the two power supplies.
Given the hastle of locating the second power supply etc why not just ditch the PSU and replace it with a stronger one?.

I have a bunch of old computers with 200 watt PSU and I was thinking of connecting all of them together to create a stronger PSU. So all I have to do is have one PSU connected to the plug, motherboard, and the other PSU connected to the PSU connect to the motherboard and the plug. Right?

I don't know whether it's possilble to combine the powersupplys but you could have it so on PSU powers the motherboard and accociated parts and the other powering fans and drives.by doing whats in the diagram.
http://coolcases.tripod.com/psu2.gif.txt

and what would happen would be a BIG BANG and lots of smoke as the voltage is a lot diffrent and the circuit tooo
i would not try this unleast u want to fry yourself and
all so the pc

backup2k1(the name u can trust)

NO, backup2k1, there won't be a big bang or smoke...

Assuming you follow the directions, the circuit in the diagram posted by DIABLO is safe. The connections indicated by the blue wires in the diagram connect the power on\off trigger circuit of the motherboard to both of the ATX power supplies, that's all. (So that one power button will switch both power supplies on and off).
Just make sure you follow the pin numbers, not the wire colors, and everything will be fine.

Originally posted by DIABLO
Not recommended for non overclockers

LOL!