A couple things to beware of if you combine the +12v and -12v to get 24V: 1) You will not have a ground lead for the 24V lines. Both wires will be at a potential different than ground and you'll let out the smoke (and possibly the fire) if either one is tied to ground. 2) The -12V feed usually has no where near the current capacity of the +12V. For example, the +12V may be rated at 7 amps, but the -12v could be as little as 1/2 of an amp. Be sure the least of the two capacities is >= to the load you intend to support at 24V.
One last thing to consider: sometimes computer power supplies rate their current capacities in non-mutually exclusive values. What that means is for example the +5V may be rated at 10 amps and the +3.3V rated at 7 amps, but the total combined current from both can't be more than 12 amps. Usually this is only on the +5V and +3.3V outputs, but I've seen it also on the +12V/+5V and the -12V/-5V.