The situation is not exactly clear to me anyway. Both AMD's web site and a third party website recommend the HD6570 and HD6670 for dual graphics support with that APU, while saying the R7-250 can be dualled with A10-7xxx series APU's. The FAQ gives some more information about how to enable it in the Catalyst software. It doesn't mention BIOS settings but I'd imagine if there is no specific setting for enabling dual graphics in BIOS, then just ensuring that the APU graphics are enabled should be enough (if the combo is indeed compatible).
The R7-250 is more powerful than the on chip HD 8670D, so if it's not possible to enable it, then just using that would probably be the best thing to do.