Well, although that rule could be useful, it is not part of the technique.
For proof, read the official description on Pomodoro, e.g. on http://www.pomodorotechnique.com/download/pdf/ThePomodoroTec.... You'll see (page 6-7) that after a first 'Pomodoro' (unit of time) work on the same task can continue into a second or third pomodoro.
