Hi there, welcome to the forums.
First, I would be very cautious about putting two GTX 560 Ti's on that PSU. It's not 80+ certified , and can only deliver 550w on the 12v lines, which are the important ones.
Basically if you are going to run a pair of 560 Ti's, you need to change your PSU. There's a reason it only came with 2-6pin connectors, because that's all it can really support.
While you may not want to hear it, you should get a better PSU before you add a second 560 Ti in. You have a combined 50a on the 12v rail, but it's a multi rail system on a low quality PSU. I suggest you read this article here: http://www.tomshardware.com/forum/306437-28-single-rail-multiple-rails-eternal-question-answered