plain and simple you can not flash the BIOS for dual channel support, this is because dual channel is either native for the Intel chip sets, as the mem controllers are located on the Northbridge .. and with AMD's the mem controllers are on the chips it self.
The main reason the BIOS has updates and therefore needs to be flashed, is because new RAM technologies come out and new CPU's technologies (i.e. a good example the whole Skt939 range) come out, adn also to increase stability of the mobo ... so the BIOS needs to be updated so that it will support the new hardware that comes out, well the new speced hardware, but not new hardware fundamentals as these stay the same on each mobo, as in this case dual channel