If you want to build a Bitcoin/Litecoin/Altcoin mining rig (or a real heavy game computer), you might consider to buy 2 PSUs instead of one very expensive. In this case, you have the problem, that the second PSU is not connected to the motherboard and will not start, when you turn on your computer.
I am not aPC expert. If you try this, you might break your PC or components. You will lose all your guarantee. When you read this, you should NEVER try this on your own machine/components!
Shortcut the green cable with the black cable (see picture). As your PSU might have different cables and/or colors, you should check this very carefully, here’s a good description link: http://www.electro-tech-online.com/threads/how-to-make-two-computer-power-supplies-work-together.88741/
Some ppl do this with a paper clip, others with a cable. Whatever you do, you need to make sure, it never slips out of the connectors.
If you want to see, how to build a mining rig with 5 or 6 graphic cards, check this article: