Both PSU's are ATX.
I'm planning on using an automotive relay (12vdc, 30A). The relay will be powered by the primary PSU via a 4-pin molex connector, and the secondary PSU will be connected via it's 20-pin connector (i'll be using a 20-pin male connector coming off the relay). This way, nobody's warranty is voided, although the secondary CPU is 3 years old, so I doubt if the warranty even still exists.
Using a relay pigtail as I have described allows me to change out either PSU without having to cut wires and re-solder on a new one, not to mention keeping the amount of useless wire in the case to a minimum.