I wouldn't actually recommend using two cheap PC PSU's, but if you want to try it, I would recommend the following:
First of all, cut the pelt leads off about an inch away from the device itself. (or less if you feel comfortable doing so) The leads on the pelt are not really adequate for carrying the roughly 19A current draw. (The manufacturers use the small leads so that they fit between the hot and cold plates.)
Run 12V and Ground wires all the way from each supply, to the wire stubs left connected to the pelt. Don't connect the two supplies together and then run a single pair of wires to the pelt. The reason for this, is that most likely the two supplies will be trying to regulate their outputs to slightly different voltages. The resistance of each set of power supply leads will help to balance the system so that one supply isn't carrying a much higher percentage of the total current load than the other.
If there is enough difference between the two supplies, one might source the whole current. Use the same length and gauge of wire (16 AWG at the smallest) to connect each supply to the pelt. Measure the voltage drop across the wire from each supply to the pelt. If one pair of wires has a much higher voltage drop than the other pair, then one of the supplies is carrying a disproportionate amount of the load.
If possible (unlikely with cheap supplies) adjust the output voltages of the two supplies so that they are as close to identical as possible. (Ideally you would do this with each supply loaded for 10A output - 1.2 Ohms.)
Be aware, that the colder your water, the greater the amount of heat leaving your pump via the water. This pump heat will limit the effectiveness of the waterchilling. It may be worthwhile trying each of your pumps on the coldside to see which configuration gives better temps.
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