"She's not pedaling " is my standard greeting on the rare occasion when we come across another tandem, its particularly fun when overtaking because you usually get an annoyed look until they see my stoker, then its all smiles.

As for receiving the "not pedaling" comment I just treat it as a hello and acknowledge them, if they wanted to be malicious they would say something else. Last week we heard a new one, someone said "look a twinner!".