"Faster! She is waiting for us."
In this case either 在 or 着 could be used, but you certainly dont need both. 着 Is kind of like -ing, but not strictly. It means that the subject is in an ongoing state (of being something). 在 Before a verb means an action is currently happening. The verb 'wait' is a strange case, because it is both a sort of action, but also a passive state, so im not totally positive which is more correct. But i know you dont need both, like duo thinks.