src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> mergeWith(ObservableSource<? extends T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return merge(this, other);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> mergeWith(ObservableSource<? extends T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return merge(this, other);
    }
Flattens this and another ObservableSource into a single ObservableSource, without any transformation.
<p>
<img width="640" height="380" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/merge.png" alt="">
<p>
You can combine items emitted by multiple ObservableSources so that they appear as a single ObservableSource, by
using the {@code mergeWith} method.
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code mergeWith} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param other
an ObservableSource to be merged
@return an Observable that emits all of the items emitted by the source ObservableSources
@see <a href="http://reactivex.io/documentation/operators/merge.html">ReactiveX operators documentation: Merge</a>