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And with +12V1 and +12V2 each having it's own regulation circuit, when combined together they can "fight" each other. They may work OK for a while, but could cause failures over the long term, as one of the regulators may fail.
I think you need to be careful with the current load when combining outputs. Usually +12V is good for many amps of current, say 10 amps or more, but the -12V side is usually less than 1 amp - they are not balanced. So for +24 volts, you need to limit the load to < 1 amp, otherwise you risk damaging the -12V output. These are designed with the lowest cost components to meet the specifications.Also some PC power supplies typically require a minimum current draw (100 milliamps?) on one of the rails (+12 or +5 V) to properly regulate the other outputs. If you neglect to have a load on the regulated voltage output, then it may not regulate other output voltages properly, or even shutdown as protective action.
For more info on base load, minimum load and balanced loading, see this:http://reprap.org/wiki/PC_Power_Supply
Good information here. FYI: I picked up a broken KO2 which needs a new tactile switch for the INPUT (SW7) in the photos. After some investigation, I think it is part AMPTCFVTR from APEM (apem.com). Now trying to get a free sample from them, as DigiKey and Mouser do not stock this part for small quantity orders.