I'm EM/IM; there's so few programs it's expected you're applying to one or the other categorical programs, and most of the categorical programs interviewed us the same day as the combined interview. I got asked why EM/IM at every interview (including the EM categorical ones - they all wanted to know why I'd waste 2 extra years) and had an answer prepared. Sounds like you have a good answer/reason for applying both. I wouldn't specifically tell them - they're gonna see it anyway, and they're gonna ask you your reasons for applying one or both specialties, so just be ready with a genuine answer.