Uhm... taking two 12V lines and tying them together for 24 volts? Pardon my possible ignorance, but I'm pretty darn sure that *does not compute*. If you crack open your power supply what you'll find (I guarantee you) is that all the lines of a like voltage come off the same fat PCB trace to the same regulator. If you tie two together at the other end essentially what you'll be doing is using a conductor that's twice as fat, that's *all*.
I think what you're thinking of is taking a +12v line and using a -12v line as "ground" thus making the potential *between them* "24v". I've heard of people doing hacks like that to power secondary voltage regulators in dire circumstances, but it's bad for the primary ones in the supply, and you can't just "tie the wires" together. You *have* to use it to drive a voltage regulator.