Spec'ing out my system I made a mistake. Like many people who haven't built systems in a while I simply added up the wattage of the devices and went shopping not really paying attention to things like 12v rail capacity. I did the math here: http://www.extreme.outervision.com/index.jsp and determined I needed 485w so I thought I was covered with a 500w PSU. I wanted quiet so I got a Nexus Breeze case (read: lots of extra fans and a controller purchased for cooling) with 500w Nexus PSU. Haven't assembled it yet but i'll be running this:
2x1GB OCZ Platinum Edition DDR500
2xWD 74gig 10k Raptors Sata II RAID 0
3xSamsung Spinpoint 250gig Sata II RAID 5
and drum roll please...
2x eVGA 7800GTX 256mb's in SLI
I already know I am going to experience shutdowns on load as I just saw a review by 3Dgameman explaining the concept of rails and the fact the Nexus 500w is underpowered in that department.
So.... rather than drop $500 on another supply or take the 15% hit for restocking the case+psu I was wondering if I could just buy a cheap power supply, stick it outside the case, and split the SLI load between the 2 PSU's. I can also power the three Spinpoints with the external PSU leaving the 2 WD HD, Mobo, CPU, 4 fans, floppy, DVD+RW, and *1* Video card for the Nexus. Does this sound like it could work???
Honestly I don't think that a good idea trying to run 2 PSU in one box like that for a MB not set up for dual power supplies (which probably wouldn't solve your issues any way).
If I was you I would just take the 15% restocking fee or purshase a seperate power supply if thats possible for that case. Also I can't see a PSU running 500 but hey that does seem like a power hungry machine.
Although it's not an elegant solution I don't see any problem in using a second supply outside the case. On our existing server (cube-style case) which I built for our company, I loaded in a lot of (parallel) SCSI drives, some of which have motors that tend to draw more power than IDE and SATA drives. Ultimately, since the case had a knockout for a second supply (backup) I installed a lower end supply just to take the load off the main PSU and provide +12V/+5V to the drives.
A little trick that might help you in powering on both of your supplies at once: I work for a video production company. We have several Macs with external RAID towers that I built. This is where our digitized media gets stored. To enable the editor to turn everything on that's needed simply by powering on the Mac I use relays. If you (or a friend) can handle a soldering iron, you can pick up a 12V relay at Radio shack and neatly solder on a drive power "wye" ('Y') cable to the relay's coil/power contacts (by snipping off one of the connectors, of course). You only need the yellow (+12V) and one of the black (Ground) wires. To the relay wiper and a normally open contact (which closes when the relay is energized) you solder a couple of wires long enough to reach your secondary power supply. Locating the motherboard ATX connector on your secondary power supply, you then attach these wires to the green wire (PS-ON) and a black wire (Ground). Plug the receptacle connector on the "wye" cable to an available tap on your primary power supply. Now, when the main power supply turns on, the +12V voltage it puts out closes the contacts on your relay which in turn ground the PS-ON connection on your secondary supply and turns it on.
A few things: Make clean, solid solder joints and cover your connections with heat shrink. Don't leave any exposed wire ends/connections that could short. Enclose everything in a plastic project box. You can then secure this to the inside of your case with double-sided foam tape. Also, make sure any wiring you pass through a case isn't going to chafe (and short) on a sharp edge (some cases don't have very finished metal edges). I should also point out that to my knowledge, from a purist (tech) standpoint, a power supply should generally be operated with loads on its various outputs. An ATX-type PSU made for a drive enclosure, for example, usually only puts out +5V and +12V. A regular ATX PSU has a numer of other voltages that likely aren't going to be used if you use it in the secondary application we're talking about. As I was told by an engineer at a PSU manufacturer those should be terminated. That's the official version, anyway. That said, I've done exactly the kind of thing you're talking about in a couple applications. At worst I suppose the PSU 'ages' faster. But I'm no expert. I only know when I used a regular ATX supply in this fashion in a couple of applications it worked fine.
This is just a suggestion for the sake of convenience. If you don't feel your up to it than stick to turning on the second supply yourself. To do so, however, you'll still probably need to connect an ON/OFF switch of some sort across the PS-ON and Ground connections I mentioned above.