IEnumerator Cor1 () {
    yield return new WaitForSeconds (3);
    Debug.Log ("Cor1: waited 3 sec");
    yield return null;
}

IEnumerator Cor2 () {
    yield return new WaitForSeconds (1);
    Debug.Log ("Cor2: waited 1 sec");
    yield return new WaitForSeconds (9);
    Debug.Log ("Cor2: waited 9 sec");
    yield return null;
}

void DoCoroutineOnObservable () {
    var a = new Subject<Unit> ();
    var b = new Subject<Unit> ();
    var obs1 = a.Select (_ => Observable.FromCoroutine (Cor1));
    var obs2 = b.Select (_ => Observable.FromCoroutine (Cor2));
    Observable.Merge (obs1, obs2).Concat ().Subscribe ();

    // 発行
    a.OnNext (Unit.Default);
    b.OnNext (Unit.Default);
    a.OnNext (Unit.Default);
}

cor1: waited 3 sec
cor2: waited 1 sec
cor2: waited 9 sec
cor1: waited 3 sec

async UniTask<Unit> Task1 () {
    await UniTask.Delay (3000); // note! Delayはミリ秒指定
    Debug.Log ("Task1: waited 3 sec");
    return Unit.Default;
}

async UniTask<Unit> Task2 () {
    await UniTask.Delay (1000);
    Debug.Log ("Task2: waited 1 sec");
    await UniTask.Delay (9000);
    Debug.Log ("Task2: waited 9 sec");
    return Unit.Default;
}

void DoUniTaskOnObservable () {
    // ストリームの構築
    var a = new Subject<Unit> ();
    var b = new Subject<Unit> ();
    var obs1 = a.Select (_ => Task1 ().ToObservable ());
    var obs2 = b.Select (_ => Task2 ().ToObservable ());
    Observable.Merge (obs1, obs2).Concat ().Subscribe ();

    // 発行
    a.OnNext (Unit.Default);
    b.OnNext (Unit.Default);
    a.OnNext (Unit.Default);
}

Task2: waited 1 sec
Task1: waited 3 sec
Task1: waited 3 sec
Task2: waited 9 sec