title: Do something after multiple observables have completed
body: <p>I'm using RXJava on Android and trying to chain together multiple API calls and do something after both API calls have finished.  My API calls all look similar to the code sample provided.  Basically make the API call, write each record to the DB in onNext, and after all records have been written, update some cache.  I want to fire off both of these calls asynchronously and then after both have hit onCompleted, then do something else.  What's the proper way in RX to do this?   I don't think I need zip as I don't need to tie together the different streams.  I was thinking maybe merge, but my two API calls return a different type of Observable.  Please let me know.  Thanks.</p>
  
  <pre><code>    getUsers()
              .subscribeOn(Schedulers.io())
              .observeOn(Schedulers.io())
              .flatMap(Observable::from)
              .subscribe(new Subscriber&lt;User&gt;() {
                  @Override
                  public void onCompleted() {
                     updateUserCache();
                  }
  
                  @Override
                  public void onError(Throwable e) {
                      Log.e(TAG, "Error loading users", e);
                  }
  
                  @Override
                  public void onNext(User user) {
                      insertUserToDB(user);
                  }
              });
  
      getLocations()
              .subscribeOn(Schedulers.io())
              .observeOn(Schedulers.io())
              .flatMap(Observable::from)
              .subscribe(new Subscriber&lt;Location&gt;() {
                  @Override
                  public void onCompleted() {
                     updateLocationCache();
                  }
  
                  @Override
                  public void onError(Throwable e) {
                      Log.e(TAG, "Error loading Locations", e);
                  }
  
                  @Override
                  public void onNext(Location location) {
                      insertLocationToDB(location);
                  }
              });  
  </code></pre>