I've been doing that for quite some time now except instead of using a switch, I'm using a
relay (SPDT) from radioshack (Part#275-0248A) and a spare molex to three pin adapter I had lying around and made it so when the first psu (400w antec) turns on, the second turns on automatically.
I have another psu (250w) that I use that I juryrigged to stay on, by using a wire. This psu powers the fans on my water-cooling tower (if u've seen the "nuclear cooling tower" bongs at overclockers.com, then u know what I mean).
Of course, you can have like a DPDT switch and relay to make it so u can turn it off or on or have it done automatically (like lights with two switches). Etc.