Most TS restaurants allow sharing (i.e., using one credit to feed two people), but Via Napoli has a strict "no sharing" policy. I've "shared" at several of the Signatures (e.g., Narcoossee's, Artist Point, Le Cellier, Yachtsman, HBD) and at some non-Signature venues (e.g., R&C, LTT, Raglan Road).
As you noted, you cannot "share" at buffets (e.g., Chef Mickey's). Additionally, you cannot "share" at family-style/all-you-care-to-eat restaurants (e.g., 'Ohana), fixed priced meals (e.g., CRT), dining shows (e.g., HDDR), or dining packages (e.g., Fantasmic dining package).