Consumir un ServiceA() que retorne una lista de objetos,por cada objeto(Mediante su ID) consumiremos ServiceB(Objeto.Id) el cual retorna detalles del mismo al final se une la resouesta del ServiceA() con ServiceB() en un nuevo Objeto.

retrofitCombinateResponses.java

//Interface API
public interface api{
@GET("{movie}/{list}")
    Observable<GenerosResponse> getAllGeneros(@Path("movie")String movie,@Path("list")String list,@Query("api_key")String key);

    @GET("{number}/{movies}")
    Observable<ResponseMovies> getAllMovies(@Path("number")int number,@Path("movies")String movies,@Query("api_key")String key, @Query("sort_by")String  sort_by);
}

//Pojo -> List Genres Movies
public class GenerosResponse {

    private List<Genre> genres;
}

//Pojo Genre -> Detail Genre
public class Genre {

    private int id;
    private String name;
}

//Pojo -> Lists Movies
public class ResponseMovies {

    private int id;
    private int page;
    private List<Result> results = null;
    private int totalPages;
    private int totalResults;
}

//Pojo -> this object is combination response the both services
public class CollectionsMovies {
    public Genre genres;
    public ResponseMovies movies;
}


 private Observable<GenerosResponse> makeRequestToServiceA() {
        return  service.getAllGeneros("movie","list","key"); //some network call
    }

    private Observable<ResponseMovies> makeRequestToServiceB(Genre genre) {
        return service.getAllMovies(genre.getId(),"movies","key","created_at.asc"); //some network call based on response from ServiceA
    }

    void doTheJob() {

        makeRequestToServiceA()
        .flatMap(userResponse -> Observable.just(userResponse.getGenres()))      //get list from response
                .flatMapIterable(baseDatas -> baseDatas)
                .flatMap(new Func1<Genre, Observable<? extends ResponseMovies>>() {

                    @Override
                    public Observable<? extends ResponseMovies> call(Genre genre) {
                        return makeRequestToServiceB(genre);
                    }
                }, new Func2<Genre, ResponseMovies, CollectionsMovies>() {

                    @Override
                    public CollectionsMovies call(Genre genre, ResponseMovies responseMovies) {
                        return new CollectionsMovies(genre,responseMovies);
                    }
                }).
                subscribeOn(Schedulers.newThread())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Subscriber<CollectionsMovies>() {
                    @Override
                    public void onCompleted() {
                    }

                    @Override
                    public void onError(Throwable e) {

                    }

                    @Override
                    public void onNext(CollectionsMovies collectionsMovies) {
                      //Object combinate whit both responses
                    }
                });
    }