Zusammen means to be together. For example: "Ich wohne zusammen mit meiner Mutter". Zueinander means to each other. For example "Wir passen zueinander".

"Zueinander" rather means "to(ward) each other" than "together". Some examples: https://www.duden.de/rechtschreibung/zueinander https://www.wortbedeutung.info/zueinander/

As per my universities's professors : 1)zusammen means together 2)zueinander means each other