No I don't believe you can and theirs no point either the onchip A6 GPU is maybe 8% (high estimate) of the 6850's GPU power making it not worth it (it would only improve the 6850 by 1-3% in CF then if it die even work). Just get the best card you can afford and don't worry about CF.
Thanks again andrewcarr. Can I ask u a question about overclocking a6 3500? I mean, can I overclock my apu with my motherboard? Is it safe? it says that a6 3500 can be overclock auto thru AMD VISION CONTROL and it shows that from 2.10ghz to 2.40ghz.
Well it does have a locked multiplier so 2.4 is the fastest you can get. This is the turbo mode top speed and I personally have never had to overclock like that. But I know of some people that have found programs to lock it in at the max turbo clock.