I have a rig that has two 200w PSU's set up this way, i didn't use any fancy connector or anything like that.  I simply shorted the green and black wire on the second PSU to make it turn on, it's connected to the second video card only.  Rig has 2x5770's.  Why waste money on another PSU when you have extras sitting around?  Just make sure that you turn on the second PSU before the main PC, as long as you get it within a second or two you will be fine.  I usually insert my paperclip and then flip the switch on the main PSU, works like a charm.