***WARNING*** connecting two power supplies together may cause serious damage to the power supplies and the equipment that they are connected to.
The master/slave setups illustrated in this thread should be okay. But, connecting two power supplies so that their regulated voltages are connected to each other, is BAD.
Background) Most power supplies use one circuit to power one transformer to make all of the various output voltages. Then, one output voltage (usually the +5Vdc) feeds back to tell the input side of the transformer what to do. The other output voltages are controlled by their relationship to the feedback voltage. (They come off the same transformer using different windings) [very over-simplified] So, the input side of the power supply regulates itself to make the regulated voltage come out right; and the other voltages drift according to what is happening to the regulated voltage.
Problem) If the two different power supply regulated voltages are connected to each other, they will make the power supplies unstable.
Lets say PSU 1 regulates it's +5 line to +5.015 volts. Let's say PSU 2 regulates it's +5 line to +5.013 volts. If the two regulated voltages are spliced together, PSU 1 will pull all the load off PSU 2 by pushing the +5V to +5.015 vdc. It's other voltages (+12V, -5 V, -12V and +3.3V) will go high. PSU 2 will reduce it's output to bring the +5V down to +5.013 vdc. PSU 2's other voltages will also go low. The motherboard and other devices will be caught in the middle of the conflict.
Real power supply sharing systems use extra circuits to prevent PSU 1 from affecting PSU 2 and vice versa.