Not meaning to be a smart-ass, but by my calculations, you'd end up with a home-brew batch of 12.5W/35.
Are these 2 different weights of the same type..ie..conventional or synthetic and from the same mfg? I could not say if that would really matter.
Hopefully we'll soon here from someone here with a high-end degree in chemistry and a background in the mfg/analysis of automotive motor oil.
Can't say about dealer oil changes. Have never paid anyone to do that.