In Assassin's Creed: Revelations, which two historical Assassins are connected through the same story?

Assassin's Creed: Revelations connects the stories of Ezio Auditore and Altaïr Ibn-La'Ahad. Ezio investigates Altaïr's legacy in Constantinople while searching for keys that unlock Altaïr's memories in Masyaf.

The game is primarily set in Constantinople during the early sixteenth century, with Ezio as the main playable historical character. Altaïr's sections take place during earlier periods and reveal important events from his life as an Assassin leader.

This structure makes Revelations a direct narrative bridge between the first game and the Ezio trilogy. It also gives Ezio a final major historical adventure before the series moves on to other protagonists.

Desmond Miles remains important in the modern-day storyline, but he is not one of the two historical Assassins named in this answer. Connor and Edward are protagonists from later entries, while Jacob and Evie Frye lead Assassin's Creed Syndicate.