ASSUMING the outputs are floating and completely independent, you may be able to put the power supplies in series (as you would with two 12V batteries). You should be able to use a mutlimeter (with the supplies turned on) to see what the voltage is between one supply's 12V supply, and the ground (or 12V output) of the other. If it's anything but open circuit, then a connection exists between the two of them.
Unfortunately, I suspect all of these will have the ground output tied to case and the ground prong on the plug, and hence be connected through the ground line, and you won't be able to do what I suggest above. A decent (current-limiting, short circuit protecting, etc.) adjustable two-channel power supply (up to 30V) will only run you around $150-$300 new, and possibly less somewhere like eBay. However, most of these will only supply 3 or 5A max, per channel (though you can connect them in series to get up to 60V, or put them in parallel to get 6 or 10A).
EDIT: Finished dangling thought