I probably shouldn't even post....,but what the hell, I will. Thier are diffrent ways to run two power supplys and as far as I am concerned, thier is only one proper way to do it. Conecting pin 14 on both power supplies should do it, in fact, cooler master makes a case that retails for 180$ (the cmstacker) that has mounting positions for two power supplies and includes an adapter to run dual power supplies that does just that, conects pin 14 via a single wire. It's kind of strange that thier is a newer version of this case out that does not have the adapter even avalable (I wonder if thier were problems with it).
I'm not an electrical engineer and I cannot substantiate if my method is right. I searched extensively for a couple of weeks on running dual power supplies. Lot of people do it by just conecting pin 14 and or pin 14 and the grounds as well as other methods. Some people do have problems though. Everything I found sugested that my method was right (it's not actually my method at all, it is the best method I could find out of other people methods). If anybody feels it is wrong, please let me know why.
The problem with just conecting pin 14 and or pin 14 and a ground is that pin 14 is a 5v leed (a low current one, but still a 5v leed). Your power supply does not provide exactlly 5v on that leed. One power supply may be supplying 4.9v and the other 5.1v or whatever. One power supply may see the voltage from the other and it can interfere with voltage regualtion. It will still probably work, but then again, you can bridge two power supplys to have power as if it were one power supply, and it will work too. That doesn't mean that it will provide acurate voltage regualtion and it causes additional stress on both power supplies that can result in premature falure (it's not going to just kill it, but it can shorten it's life). Further it just causes some power supplies to shut down.
Second problem is that switching power supplies require load to function properlly. While a few power supplies do not on some or all channels (and I am guessing they provide thier own load), most do. You can look on the label of many and they list both a minimum and maximum load. Thier is a minimum listed because they must have the load to oppperate properlly. Without the load, some will just shut down, others will provide very poor voltage regulation and may give eratic performance.
So what is the solution? It is simple, and very cheap. You need a relay, and 1 or more resistors. When your psu is conected to a motherboard, it does ground pin 14 to turn on the power supply. Take power from a regular 4 pin molex from the first power supply like you would plug into your hard drive (12v relays are the most common, thats the yellow wire and a black wire from the first power supply) and run them to the relays switching power. run pin 14 and a ground (I think that I am using pin 5 but any ground pin will work) off the 20 pin motherboard conector to the relay switching circuit. Thats it. It will cause the first power supply to turn on the second power supply without crossing the two power supplies power lines, using a couple dollar relay like this
I know that they make inline relays (just wires coming out so you just conect the right wires) but radio shack doesn't seem to have them (I used radio shack cause they are common/easy to find, though over priced and have a crappy selection).
Problem 2, load on the power supply. It's even simpler to fix, and not doing it will cause some power supplys to shut down, and will cause poor voltage regulation on others. If you are using the second power supply for drives, you are probably fine on the 5v and 12v rails. Drives (hd or cd/dvd) use both 5v and 12v and will load both channels. If you are only using the second power supply for lights, fans, pelt chips etc. (that generally use 12v), then you are only ok on the 12v. You need to load the 3.3v channel and if not running drives, the 5v channel. Thier are two ways to do this.
First would be to just use a simple resistor like this (WARNING, you have to get the appropriate size resistor, not this one, I linked to this just for the picture and price)
The math to calculate the right resistor is simple and I can help if anyone needs it. Frys has/had 2 packs of appropriate resistors for about 99 cents for 2 and radio shack had the same for about 2$ (in store a couple of years ago when I built my adapter).
The resistors produce heat and should therefore be mounted to something metal (I have heard of people doing things as simple as zip tying them to the vents on the side of the second power supply.
Opps, forgot the second way (edit)
You can hook fans or other similar devices to the 5v and or 3.3v. hooking a 12v fan up to a 5 volt leed will not hurt the fan, but it will reduce the performance of the fan and thierfor it's cooling. I have a couple of 5v fans that I gutted from these
Compusa was clearing them out for 5$ and I figured the fans were worth 5$.
You can volt mod fans with resistors or something like that to counter it too but I have never done it so I'm not sure how it works (I could try to find som links if someone requests it). Basically anything that will put load on the channels. Look at the ratings on your power supply though. Some require a pretty heavy load (a few amps) on the 5v that fans might not provide.
Like I said, I'm not an engineer, but after researching it a lot, it seemed the proper way to do it and all the reasons for doing it made sence. Further, I made a little plug an play module that I have plugged a couple of power supplies into, and never had a problem, with crapy power supplies like a deer 350 watt, as well as good ones. I'm currentlly running a mad dog 250 watt as my second. Right now it is just powering the pelt on my vid card, but after I do a little rewireing, it will run fans and light too.
Just my 2 cents worth, tell me what you think.