Actually there are reloading recipies (particularly in the Lyman manuals) for 9mm down to 90gr and .380 up to 115gr, so you certainly CAN use the same bullets for both, but you're sort of limiting yourself like this as one or the other isn't going to be optimal (and for 9mm I prefer 147gr, which absolutely will not work for .380 at all).
So yes, it CAN be done. You probably don't want to do it though.