One Mysql Table with Multiple TIMESTAMP Columns

MySQL versions before 5.6.1 would not let two TIMESTAMP columns in the same table, unless as you rightly noted with out defaults and allowing null.

MySQL 5.6.+ allows two or more TIMESTAMP columns in a table.

More here: http://shankargopal.blogspot.in/2013/03/mysql-566-timestamp-columns-and-default.html

It's documented in the MySQL docs:

In addition, you can initialize or update any TIMESTAMP column to the current date and time by assigning it a NULL value, unless it has been defined with the NULL attribute to permit NULL values.

http://dev.mysql.com/doc/refman/5.0/en/timestamp-initialization.html