If you have multiple observables that you want to execute simultaneously, and be notified when they've completed, you use the Observable.forkJoin()
function, passing it an array of Observables:
const observable1 = this.http.get('url1.html');
const observable2 = this.http.get('url2.html');
Observable.forkJoin([observable1, observable2]).subscribe(results => {
const results1 = results[0]; // observable1 results
const results2 = results[1]; // observable2 results
this.processResults1(results1);
this.processResults2(results2);
});