http://www.add2psu.com/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.
There are two versions available, which one should I use?