My guess is that he was suggesting the 560 to be used as a PhysX card, but that's not even necessary with a 670 (a powerful card that can handle PhysX EASILY on its own and a 560 is WAY overkill for that anyway). In fact, most newer mid to high end Nvidia cards can handle PhysX on their own quite easily.
Regardless, you certainly couldn't use the 560 in SLI with the 670.
Now, how much horsepower do you REALLY need? Are you running a multi screen setup or a single SUPER high res monitor? If not, you don't really NEED SLI, even for BF3. A single 670 will chew right through it on Ultra @ 1080p.