it is sort of a crossfire. iirc it used to be called asymmetric crossfire or hybrid crossfirex now it is called amd dual graphics.
the highest card for dual graphics is 6670.
your discreet gfx card (6790) is much more powerful compared to the integrated gpu inside the a8 3850.
since amd doesn't allow combining a8 3850 and 6790 (imo if it did, the 6790 would lose performance), the dual graphics option doesn't show up.
thank you, case close