Updated Clarification
If your asking if you can combine the APU and 6670 in CF then no you cannot combine graphic of APU and GPU for better performance.
Now if your asking if you can still use the APU graphics on one monitor and the GPU on another monitor then yes you should be able to do that.
Okay, so for others that have come across this later on, as I have, here is some solid info. There is a lot of different suggestions and information being thrown out here, many of whom have said they do not for for certain but are pretty sure, which can become a lot to try and bring together for a final decision. Here are the straight facts, coming from my own research, experience, and info directly from AMD.
* You have to make sure that your GPU matches the graphics on the APU for dual graphics (hybrid crossfire).
* You also have to ensure you get the drivers for the APU, NOT the GPU, and set it in the BIOS to run dual graphics (hybrid crossfire) as well as CCC.
* AMD does NOT support tri-crossfire or any such thing. If it IS possible, I would not suggest it. Better to just get a dedicated CPU (FX 8320 for example or core i7) and crossfire two dedicated graphics cards. Playing with this stuff and OCing does wear down your computer and decrease its overall life, not to mention you could totally botch it up and fry your machine. I do not OC anything and run any game on ultra settings with amazing performance, without spending over $600, so I know that it is not absolutely necessary.
* Finally, the quote above is misinformed. You CAN dual graphics (maybe not crossfire, if you want to get technical) an APU and GPU for better performance (though the current availability of APU's with worthwhile graphics is not that great). You CANNOT have a monitor plugged into the APU graphics and one in the GPU at the same time. You can do that with some performance issues, but they will NOT act independently. You choose one or the other; dual graphics with APU and choose which you plug into, or a dedicated graphics card that works independently from the APU.