Formal:
* Lire ensemble: This is the most common and formal way to say "read together." It literally translates to "read together."
* Lire à haute voix en commun: This means "to read aloud together."
Informal:
* Lire à deux: This is a more informal way to say "read together," literally meaning "to read in two."
* Lire en duo: This is similar to "lire à deux" and means "to read in a duo."
* Se partager un livre: This means "to share a book" and implies reading it together.
The best way to choose the right phrase depends on the specific situation and your level of formality.