List<Observable<Contributor>> observables = new ArrayList<>(2);
    observables.add(getSlowCachedDiskData());
    observables.add(getFreshNetworkData());

    Observable.concatEager(observables)
        .subscribeOn(Schedulers.io())
        .observeOn(AndroidSchedulers.mainThread())
        .subscribe(contributor -> Log.d("TAG", "COMPLETED!"));