if two people are learning from an untranslated shas, will they already know and share all the information on any given point that is contained within an Artscroll shas?
Not at all. One of the most valuable qualities of AS Shas are the notes which do a lot of research on numerous points on each daf. To do this research even for an accomplished Talmid chacham would take many hours. Also practiaclly all the rashis are explained. Every now and then there is a shver Rashi. A S is an invaluable tool, even if not used for translation of pshat in the gemara.