Do two layers of 1/2" drywall provide a fire rating equivalent to one layer of 5/8" drywall?
Just to shed common sense on this.  Almost in all cases is it cheaper and less of a hassle to demo the 1/2" and install 5/8" than to install 1/2" over 1/2".   So to answer the question, just install the 5/8" so you aren't having a philosophical debate on fire rating with a local inspector.