NOT just sequels. Films that make a good pair for more interesting reasons - maybe they share an actor, a setting, a theme, or just bounce off each other in an interesting way.
My suggestion is The Town and Good Will Hunting. Obviously Ben Affleck is in both (and directed and co-wrote the other), and they're both set in working class Boston (Charlestown and Southie respectively), but they also share a lot of themes, about loyalty, escape, and how people's upbringing shapes who they are. And they're both great.