If the card you buy has 2 6 pin connectors on it you need to use both, there are rare occasions where people have gotten by without it, but it is unlikely and can actually cause damage so don't risk it.
The 7870s im seeing on newegg need 2 6 pin connectors so you are going to need to get a PSU with 2.
Yeah, I think you are right. I don't know why amd has written that information if all the 7870 that are avaiable in the market show that they need 2 6 pin connectors. Thanks for your help!
PS: sorry for any mistakes