def add_dicts_by_key(in_dict1, in_dict2):
    """
    Combines two dictionaries and adds the values for those keys that are shared
    """
    both = {}
    for key1 in in_dict1:
        for key2 in in_dict2:
            if key1 == key2:
                both[key1] = in_dict1[key1] + in_dict2[key2]
    return both