It actually came with 1 cable with 2 6+2 connectors in the other end. (I guess that's 2x 6+2)
So.. would it be fine to just use that one?
I mean, if it's supposed to be able to handle 2 GPUs, why not the two connectors of one, right?
Then you got shorted a cable since since all quality 750watt's come with 4 connections. I would contact retailer http://www.newegg.com/Product/Product.aspx?Item=N82E16817182264
specifies 4 x 6+2 so that should have been two cables.
Yes you are fine using both connectors on 1 cable.