def merge_two_dict(dict_x, dict_y):
    """
    :param dict_x: {'a': [3, 4], 'b': [6]}
    :param dict_y: {'c': [3], 'a': [1, 2]}
    :return: {'c': [3], 'a': [3, 4, 1, 2], 'b': [6]}
    """
    dict_z = dict_x.copy()  # Never modify input param , or take inplace as param for explicit use case
    for key, value in dict_y.items():
        if dict_z.get(key):
            dict_z[key].extend(value)
        else:
            dict_z[key] = value
    return dict_z