Hi niko, Michael did mention 'surface to surface', so if have a sphere intersecting a cube both are solids and that method of filleting won't work for that situation, but if you separate the cube into individual surfaces you can then use surface to surface filleting and then join everything back to a single solid body.
Usually in that situation you would Boolean union the solid bodies then apply the fillet using the intersecting edge, either way works.

Cheers
Danny