Hooking the supplies up in the manner they're prescribing will dead short the power supply, it won't even turn on =\ PC power supplies are short circuit protected so they'll spend hours trying to figure out why the supply is going into protection mode. Apparently you're smarter than your friends =P
As a side note, most PC supplies have a -12 volt rail for things like RS232 circuitry so if you run the +12 and -12 you can get 24 volts, but typical PC supplies will only provide a few hundred ma's on the -12 rail so that's all you'll get as you're limited by the supply line with the lowest current rating, even if the +12 line is rated for 10+ amps.