I finally tried Dual Graphics on my AMD Kaveri A10-7850k based system. Kaveri A10-7850k is an onboard R7 250 GPU, and pairs with any standalone R7 250 card in Dual Graphics mode. However, the standalone card must be DDR3. The card I am using is MSi R7 250 2GB DDR3, and is less than $60.
Popping it in, the system immediately recognized it, and AMD Catalyst asked for setting Dual Graphics mode. Easy..
GPU-Z shows Dual Graphics