def merge_dictionaries_summing(first_dict, second_dict):
    """Merge two dictionaries summing values with the same key.

    Args:
        first_dict (defaultdict): A defaultdict dictionary.
        second_dict (defaultdict): A defaultdict dictionary.

    Returns:
        The enriched version of the first dictionary (it works in place). The
        returned object is of type `defaultdict`.

    Note:
        This only works only with two input objects for the moment. Later
        improvements could be made, if needed.

    """
    for k, v in second_dict.items():
        first_dict[k] += v
    return first_dict