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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.
For more info on base load, minimum load and balanced loading, see this:http://reprap.org/wiki/PC_Power_Supply
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.
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.