There is a trick that can be used to achieve the desired result.
The trick involves making four variants of the glyphs. Bold Upright, Black Upright, Bold Slanted, Black Slanted. Make them so that only the weight/width changes from Bold to Black Upright, and only the angle changes from Bold to Bold Slanted and from Black to Black Slanted.
Then give both axes the same name (in my example file “Combined”, and give the first the axis tag “wght” and the other one “wgh2” (doesn’t really matter).
Now comes the tricky part, open the exported VF in a hex editor and change all occurrences of the letters “wgh2” in the binary file to “wght”.
Now you have two “wght” axes, which will be applied at the same time. You can test the result yourself with the attached file.
This method is called “Higher Order Interpolation” and was discovered by Underware.
HigherOrder.zip (5.3 KB)