I am having a similar issue. I have an Nvidia GT 555 M and the integrated graphics. I have it set to run Dolphin with the Nvidia and it works fine with Dx9 and OpenGl.
However, when I play my Fire Emblem Radiant dawn game, there are glitches with graphics unless I use Dx11, but when I use Dx11, Dolphin does not see my Nvidia Gt 555. If I switch to either of the other backends, it sees the nvidia immediately, but not for Dx11.
However, when I play my Fire Emblem Radiant dawn game, there are glitches with graphics unless I use Dx11, but when I use Dx11, Dolphin does not see my Nvidia Gt 555. If I switch to either of the other backends, it sees the nvidia immediately, but not for Dx11.