Shituf in a nutshell
Shituf is the Hebrew word for partnership. In Judaism, it means belief in the G-d of Israel, plus an independent will that is seen as one’s personal salvation.
The rabbinic term for one who believes in shituf is a min, which, in Hebrew, means a type. Rabbi Yaakov Emden, z”l, defines a min as a member of a sect that believes in two powers.
With that foundation, one can discuss Christianity, Islam, and rabbinic Judaism. I say that all three are shituf.