I have been studying this topic last week, since I am planning a similar build.
There is two things that need to be secured:
- The secondary PSU as to be switch on, example by the ON signal from the MOBO.
- There must be a solid ground connection between the two PSU's.
The "adapter" you have linked to solved the first problem.
The "adapter", simply switch on the secondary PSU by enabling the ON signal with a relay, the relay is switched by the primary PSU (5V or 12V).
But you can do this "for free" by simply take on signal from primary PSU and connect it to secundary PS, here are the shared connections:
http://www.corsair.com/en/blog/testing-your-corsair-power-supply/(the example shows how to run a PSU, without MOBO, this is just to show the two signals that is needed to power on the PSU, the signal could as well come from the MOBO where the primary PSU is connected).
Here is is a guy who is selling that solution:
http://litecoinminingparts.com/shop/cables/dual-psu-power-supply-adaptor-cable/(you can build it yourself, if you want to save $20)
From an electrical perspective it is important to have a solid ground between the two PSU's.
An ATX PSU has actually ground on chassis, so if mounted in same cabinet is a very solid ground established.
You can also just connect a solid ground wire between the two chassis (e.g. a 2.5 mm2 wire).
The system will properly work without the "extra" ground connection, but then will the ground be established through the MOBO, which is a relatively long path, which potential could give you a stability problem.
