Sorry, but this is just not right. SSGI completely overrides the AO. At least the SSAO effects, and DFAO is just a different way of approximating the same effect - indirect shadowing. When you enable SSGI you immediately see the SSAO effect go away, as expected. So the fact DFAO and SSGI can work together is way more of a bug than a feature.
In traditional rendering, the AO was originally invented as a means or providing indirect contact shadows in areas which would be otherwise uniformly lit by an ambient lighting. It’s a remnant from the times when ray tracing skylight illumination was expensive even in offline rendering.
Then, AO became something a bit different. It was often used in rendered which had really terrible, low accuracy global illumination solutions such as Final Gather in Mental Ray, where AO was used as a means of “detail enhancement” where the FG would take care of general indirect surface illumination while AO would supplement the more accurate local indirect shadowing. But this whole approach was not very physically based.
This has spawned a gross trend where people would tend to just render AO pass and multiply it over final image. And they would do it even without actually knowing the reason. Many people would just render an AO pass and multiply it over the the beauty pass just because they learned it somewhere.
Once better GI solutions appeared, mostly using brute force path tracing for primary bounces and some sort of radiance/light cache for secondary ones, the indirect illumination contact shadowing was no longer lost due to the inaccuracy, so multiplying an additional AO pass on top of the beauty image actually made it look worse and less realistic. The interior visualizations for example would look like there’s a layer of dirt/soot in most of the concave areas.
And this trend unfortunately made it in games too, where people think that the more knobs they turn on, the better and more realistic will their scene look. And that’s just no true. If you enable DFAO alongside SSGI, you will get the benefits of SSGI compromised by the inaccurate, blurry artifacts of DFAO. SSGI actually approximates local indirect bouncing, so if you have a cube lit by sun and sky, on sunny side, the concave areas will actually glow, where as on the shadowed side, there will be subtle indirect shadow. DFAO on the other hand just adds the ugly, blurry dark blobs even on sunny side where they do not belong.