It might be somewhere else too, but in Rule book page 10: Under Double Commands Cards:
"Note that a single model may still only Move and Act once during a single overlord turn."
A model can only be activated once during a turn. Note that the game is divided by into rounds. You can activate a model using an Interrupt card between 2 hero turns, and then later during the overlord turn. The Interrupt card creates a mini overlord turn in the hero half of the round.