src/main/java/io/reactivex/Single.java
@BackpressureSupport(BackpressureKind.FULL)
    @CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Flowable<T> mergeWith(SingleSource<? extends T> other) {
        return merge(this, other);
    }
@BackpressureSupport(BackpressureKind.FULL)
    @CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Flowable<T> mergeWith(SingleSource<? extends T> other) {
        return merge(this, other);
    }
Flattens this and another Single into a single Flowable, without any transformation.
<p>
<img width="640" height="415" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.mergeWith.png" alt="">
<p>
You can combine items emitted by multiple Singles so that they appear as a single Flowable, by using
the {@code mergeWith} method.
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The returned {@code Flowable} honors the backpressure of the downstream consumer.</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code mergeWith} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

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