bb2k, I just skimmed these posts so if someone already answered this, then I apologize. The reason you get 13v on crossing two PSU's @ 12v each, is because computer PSU's are switching power supplies. By connecting the outputs of two supplies, you put both inductors in parallel, there by changing their two individual values into one different value, and thereby change the values present in the LCR series time circuit. Not to mention changing the resonance of the incoming AC ripple by bleeding this 'new' value back through the kickback diode, and changing the value of the output capacitors. So, for an arbitrary value, let's say each PS is using the same 100uH coil. You take both of these, and since they're in series: L^t= [(1/L1) + (1/L2)] (I'm not including mutual inductance here on purpose) so if L1 and L2 equal 100uH we get L^t=1/50 uH which changes the output value of the LCR, but just enough to screw up the compensation by the output capacitors. So the capacitors do their best at an attempt to limit the output to 12v, which they can't because the values have changed, which the monitoring circuit doesn't know, so it keeps sending pules of a specific width and power, and it becomes impossible for the monitor to accurately control the voltage because so many variables have changed. Not only that, but the monitor can't even 'see' the new voltage because of its location within the circuit.
Also, if your PSU design included an output voltage limiting comparator (fed off of a reference voltage) the reference voltage would be wrong as well, because of the changed values of the inductors in relation to the input transformer. So anyway. Just thought that might help.
One thing of note on your UCC29002/39002 chip. It's designed for about 10mA. So, suffice to say, it would be severely lacking for this project. (that'll be the day... "10A of cross-regulation from a .5" SMD package!!!")
