const users$ = from(['Jeanne', 'Serge']);
const userMessages$ = (name) => from([`${name}: hola`, `${name}: hello`]);

// Flatening version
users$
  .pipe(
    mergeMap(user => userMessages$(user)),
  )
  .subscribe(message => console.log(message));

// Subscribe in Subscribe version
users$
  .subscribe((user) => {
    userMessages$(user)
      .subscribe(message => console.log(message));
    });
});

// Both log :
// Jeanne: hola
// Jeanne: hello
// Serge: hola
// Serge: hello