I still get A LOT of people asking about how to turn on their second PSU.
Manual switch? Tie the green wires together on the main ATX connector?
None of that. It's simple.
Take a female Molex and wire it up to a simple automotive relay. Like the kind you would use to turn on a set of fog lights.
These are $5 at the local autoparts store or all you can carry at the local junkyard (80's VW's have a half a dozen per car.)
Just wire the Molex's +12V and ground to the relay and plug that female Molex into the male Molex on PSU # 1.
Take the green and any black wire from PSU # 2 and hook it up to the switched terminals of the relay.
When the relay is energized by PSU # 1, the circuit between the green and black will close causing PSU # 2 to turn on.
If you want something really clean looking, use a fan header on the board to energize the relay!
If you have a modular PSU with all modular connectors like an Ultra or a Silverstone Strider, just leave the main ATX cable off the PSU altogether and only connect up the peripheral connectors you need. Buy the pins for an ATX connector (http://www.performance-pcs.com/catalog/index.php?main_page=product_info&cPath=174&product s_id=3592) and only run two sleeved wires from the would be green and black leads to the relay (heatshrink over the pins, please.)
Talk about clean looking!