“Subject to the provisions of this section this chapter relates only to employment by the one employer. (2) If a trade or business, or an undertaking (whether or not established by or under an Act), is transferred from one person to another -- (a) the period of employment of an employee in the trade or business or undertaking at the time of the transfer counts as a period of employment with the transferee, and (b) the transfer does not break the continuity of the period of employment.”