It is absolutely possible. In the poster's case, she did activate her pass for the day at DL, on her phone. As she said, APs can enter using the app. By the time her DH got to the FP machine in DCA, her pass was already activated by her for the day. It would be no different than if he went into DL with them and turned around and went over to DCA to do the same. This just saved them steps. It wouldn't matter what kind of FP they were getting (show vs. ride) - it all works the same. And you can hold FP for both parks at the same time.