//Service GetStudent
public Observable<ResponseService> GetStudent(String id_student) {
        return retrofit.create(Service.class).GetStudent(id_student);
}

//Ids of Students
String id_student1= "asdfghjkl11";
String id_student2= "qwertyuio88";

//creation of observables with the service
Observable<ResponseService> student1 = GetStudent(id_student1).subscribeOn(Schedulers.io());
Observable<ResponseService> student2 = GetStudent(id_student2).subscribeOn(Schedulers.io());


Observable<List<ResponseService>> zip = Observable.zip(student1, student2, (d, r) -> {
            return Arrays.asList(d, r);
        });

zip
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new Subscriber<List<ResponseService>>() {
            @Override
            public void onCompleted() {

            }

            @Override
            public void onError(Throwable e) {

            }

            @Override
            public void onNext(List<ResponseService> ResponseServices) {
                //Result of list request :D yea!
            }
});