public class MainActivity extends AppCompatActivity {
    private final String TAG = this.getClass().getSimpleName();
    private CompositeDisposable mCompositeDisposable = new CompositeDisposable();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button button = findViewById(R.id.btn);

        Observable.merge(obs1, obs2)
                .subscribe(data -> Log.d(TAG, String.valueOf(data)), throwable -> Log.d(TAG, "throwable"), () -> Log.d(TAG, "Completed"));
    }

    Observable<String> obs1 = Observable.interval(1, TimeUnit.SECONDS)
            .map(data -> "obs1: " + data);

    Observable obs2 =
            Observable.just(1, 2, 3, 4, 5, 6, 7)
                    .takeWhile(data -> data < 5)
                    .map(data -> "obs2: " + data)
                    .doOnComplete(() -> Log.d(TAG, "done"));

}


LOGS:
D/MainActivity: obs2: 1
                obs2: 2
                obs2: 3
                obs2: 4
                done
D/MainActivity: obs1: 0
D/MainActivity: obs1: 1
D/MainActivity: obs1: 2
D/MainActivity: obs1: 3
D/MainActivity: obs1: 4
D/MainActivity: obs1: 5
D/MainActivity: obs1: 6
D/MainActivity: obs1: 7
D/MainActivity: obs1: 8
D/MainActivity: obs1: 9
D/MainActivity: obs1: 10
D/MainActivity: obs1: 11
D/MainActivity: obs1: 12
D/MainActivity: obs1: 13
D/MainActivity: obs1: 14