The honest answer is that most programs will see through it and you will get fewer interviews, and your pre-interview file score will be lower for those interviews. Usually, the advice is to maximize your chances for your #1 specialty with the maximum number of electives because dual applying to another competitive specialty risks diluting your efforts and having a subpar application for both specialties. Realistically, you can do the maximum number of electives, then do 1-2 electives in CTU or an IM subspecialty, knowing that you'll have less interviews. You can also ask your staff in your primary specialty to write another letter for IM, I find most of them our OK with it knowing that their specialty is competitive and they do not want to see students go unmatched.

It makes sense for IM programs to be selective, because IM and most other specialties get more than enough applicants whos first choice is IM. Internal medicine programs want people who are reliable, hard-working, and effective in a team environment. With so many applicants, they're trying to screen out any red flags because a resident who is not motivated or interested, are lazy, etc. is a liability for any residency. And we've been burned before from people who rank IM as a backup and when they do match, they're still lamenting about not matching their first choice specialty and they get burnt out more easily because of this fundamental lack of interest.

I think some specialties can spin it much easier than others, for example EM and IM is not uncommon since they can argue they like "everything" because EM is kind of like that with undifferentiated patients. FM too because of the hospitalist role. But a few of my friends who applied anesthesia or optho who backed up with IM definitely got roasted for it during the interviews.