join

Observable<String> left = 
        Observable.interval(100, TimeUnit.MILLISECONDS)
            .map(i -> "L" + i);
Observable<String> right = 
        Observable.interval(200, TimeUnit.MILLISECONDS)
            .map(i -> "R" + i);

left
    .join(
        right,
        i -> Observable.never(),
        i -> Observable.timer(0, TimeUnit.MILLISECONDS),
        (l,r) -> l + " - " + r
    )
    .take(10)
    .subscribe(System.out::println);