//For every `T.getId()`, fetch `ArrayList<E> anotherList`
for(int i=0; i<arrayList.size(); i++){
    T object = arrayList.get(i);
    fetchData(T.getId());
}

retrofit.fetchData(T.getId())
        .subscribeOn(Schedulers.io())
        .observeOn(AndroidSchedulers.mainThread())
        .flatMap(...)
        .merge(...)
        .subscribeWith(...)