This is all you need.
Follow the instruction videos on their website or YouTube and you can have it set up in about 60 seconds...
There are cheaper or DIY ways to use 2 PSU's but they require cutting wires / using paperclips to connect wires etc, which was a little sketchy to me, so I spent the $13 to get a properly designed device.
I've used the cheaper one (green, $12.95) on my rigs as the newer one wasn't available when I purchased it.
All I do, is plug the PSU's in according to their videos and it works fine. I do make a habit of turning BOTH PSU's on and off at the same time. I'm not sure if I actually need to do that, but better safe than sorry I suppose (I imagine I don't need to do this and it's what the actually add2psu device does, but hey... )
The newer one I believe allows the power to keep flowing for a while after you have shutdown, which allows the fans to keep spinning and cool things down in the event of a sudden shutdown. Read their website about the differences, but I can confirm the cheapest one works for what you want.