That's what I was confused about in your post. I don't really know how you'd use two power supplies with the printrboard. I'm just powering the whole thing with the 12 V lines.
I've written up a little about how I handled it (with your help!) here:
http://ei8htohms.tinyparts.net/?p=546
I've written up a little about how I handled it (with your help!) here:
http://ei8htohms.tinyparts.net/?p=546