def split_dict(dict_, set_):
    """
    Partitions a dictionary into two disjoint sub-dictionaries.
    
    :param dict_: The original dictionary.
    
    :param set_: A set of keys used to form the disjoint dictionary.
    """
    new_dict = {}
    old_dict = {}
    for key, value in dict_.items():
        if key in set_:
            new_dict[key] = value
        else:
            old_dict[key] = value
    return old_dict, new_dict
