In a two storey house, is 5/8" drywall required on the ceiling of both the first floor ceiling and the upstairs ceiling? Or is it only required between the second floor ceiling and attic space as a fire separation?
Thickness of gypsum board has nothing to do with upstairs or downstairs. It has to do with spacing of supports: ceilings = 1/2" on 16" on center and 5/8" on 24" on center. (A 1/2" board will sag over time from support to support if it spans too far.)
Likewise thickness of gypsum board has nothing to do with "party walls". The Building Code requires fire wall SYSTEMS. There are many systems that use 1/2" Type X and some that use 5/8" Type X. It has to do with spacing of supports, nailing pattern, and use of insulation.
By the way, the Building Code also requires a minimum sound rating rating in "party walls" too. Often we'll use the sound wall SYSTEM criteria as the governing criteria, and just specify that the board be fire rated (Type X) and kill two bird with one stone.
Now, if the space is in a moist environment, (porch, steam bath area, etc.) we'll bump up the thickness for ceiling gypsum board, (I.e.: 1/2" to 5/8" thick on supports at 16" on center.)
I am Contractor in Massachusetts, learned my trade in Vermont from old timers. I have been thought to build things like you mean it, though. Gentleman that took me and thought me how to renovate older homes never believed in 3/8 - 1/2 drywall for interior walls or 1/2 inch drywall for ceilings and exterior walls. I have been on my own since 2001 and I have been always using 5/8 on the ceiling and all of the exterior walls as well. Interior walls I use 1/2 inch. Bathrooms roxul insulation and then quateboard. Inspectors were more then satisfied.
Don’t be afraid to go above minimum code use 5/8 drywall or blue board insulate your bathrooms, buy quality doors and be happy.
That I am aware of, 5/8" is required only in garages or perhaps other areas where there are flame spread issues. Only on walls and ceilings that neighbor living spaces.
I am only guessing but condos and apartment buildings will require it on ceiling between floors of living spaces. In single family homes, I have never seen it applied like your question asks. It can be used for firestopping in concealed places required by code.