Similar questions have been asked here before:
But my example seems a bit different since I do have have a short Schengen visa before. I'm a Chinese citizen and have a used Spain long stay study visa (type D, multi-entry, stay 180 days) that's valid until Apr. 20th. What I want to know is if I can start applying for a short stay visa before the old one expires (i.e. Apr. 20th)? The two visas' valid date do not overlap: I want the new short stay visa to be valid from, say, May. 1st (which is just after the old one has expired).
The answer here says yes. However, in that example while the old one is multi-entry like mine, it does not specify what the new visa type is. Is the example still valid if the old visa is long stay, multi entry but the new one is a short stay, single entry?