Midwest2, like YitzchokM pointed out what I said about Christianity and Islam is exactly correct about those gentiles. But it, obviously, isn’t the case for Jews — for whom both are A”Z.
The Rambam’s Psak, as noted above, is what we rule with today. The vast majority of Ashkenazic poskim didn’t address the issue altogether due to the Church censoring of Seforim. Imagine the consequences should a Sefer published in Europe have stated what the Rambam paskened about Christianity.
In any event, the Noda B’Yehuda, one of the Ashkenazic greats, explicitly rules that shituf is pure A”Z when practiced by a gentile.