Would it be all right for me to nominate "Detective Conan & Magic Kaito" as a single fandom? They're part of a canon shared universe. Jointly, the fandoms call the universe and umbrella fandom DCMK, but that abbreviation is pretty obscure to people who aren't in it.

(Though there's also a third series (Yaiba) that seems to be part of the shared universe as well, but it doesn't share the massive fan overlap of the other two, and generally doesn't get included.)