it's called dual graphics... as to what can; supposedly any r7 gpu will work...
that said the best results are apparently with the r7-250... which apparently is the closest gpu in performance to the on-board graphics. That said with the faster the ram you can get ahold of the better it will dual graphics with the stronger cards in the r7 family. In theory the closest GPU around to the onboard graphics is the hd 7770... which on paper has the same statistics... so it SHOULD dual graphics just fine with the a10-7850k. That said the gddr5 ram on the 7770 makes the 7770 almost 40% faster in benches... so as i said before the closest match is likely the r7-250
the previous poster is right, it will dual graphics with the r7-240...