I'm not sure but I think dual graphics mode is for dual monitors. Not pairing cards like sli crossfire and stuff. And I'm sorry to be too lazy to look it up for you but you might want to see if those GPU's can work paired ( I don't think they can).
dual graphics is what amd calls it when you put a graphics card and one of there apus in crossfire. I am pretty sure you they can be paired at least that is what it is advertised as.