In Hyrule Warriors, 2-player mode was unavailable in Story Mode, but it was available in all Adventure Mode maps. All I had to do was push + on the second player's controller during the battle info screen. In Fire Emblem Warriors, however this doesn't seem to be the case.
Once I got to Chapter 3, an icon popped up in the top-right part of the screen that said "P2 Can Join" which was refreshing to see in the main mode. However, pressing + or pushing in the left stick on the second controller didn't change anything. None of the buttons on P2 activated co-op!
What am I doing wrong? How do I activate 2-player mode?