MySQL - 5.6.5 2, 2 TIMESTAMP column / CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP

sql

Why there can be only one TIMESTAMP column with CURRENT_TIMESTAMP in DEFAULT or ON UPDATE clause?

in MySQL 5.6.5 (2012-04-10, Milestone 8) is possible to have 2 fields, as follows: 

CREATE TABLE `foo` (
  `id` INT(10) UNSIGNED NOT NULL,
  `created_at`  TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `updated_at ` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
) ENGINE=INNODB;