http://hexus.net/tech/reviews/cpu/46157-amd-a10-5800k-dual-graphics-evaluation/
Yes, its called dual graphics and this review shows an example using the 6670 as you wish to
Moto
What do I mean is, I will use the dual graphics and at the same time I will add another one discrete card hd 6670 (aside from one discrete card used for dual graphics) to crossfire them. Hence, there are two hd 6670 discrete card will be use plus the APU. Is it possible?
That one I'm unsure on, I would suspect you can either use the Apu+1 card, or crossfire the cards and it disables the Apu gfx, maybe someone else knows more on this but if not, you can always try mailing Amd to clarify,