now, if aPD is the program director where you applied... that would just take the cake.
Before we start any crazy rumors, I am NOT the PD of the program he is talking about. I ain't med-peds. So that's that.
Looking at all the advice on this thread, let's look at it from both the Med-Peds and IM residency PD positions. Remember, what we are looking for is some combination of:
1) Enough smarts to practice clinical medicine safely and well
2) Enough drive to get the work done
3) "commitment" to the field = unlikely to transfer out in the PGY-2 year, and unlikely to be using IM as a backup for a ROAD application (because we don't want to waste interview slots on someone who is unlikely to match)
4) A good fit for the program (hard to define, but that you'd fit well with our culture)
5) Lack of a personality disorder
In general, we use your application to decide #1-3, and the interview for #4-5.
So, how will each PD involved see you situation? The only point for discussion is #3 (all else is the same whether you apply to 1 or 2 programs). The Med/Peds PD will say: "Gee, this guy is applying to both IM and Med/Peds. I worry that he's not committed to Med/Peds, and may simply drop out and switch to straight IM. Hence, I think I'll rank him lower."
The IM PD thinks: "Hmm, he applied to Med/Peds. Well, either he matches with me or he matches with Med/Peds. If he matches with me, it's extremely unlikely he'll try to switch to med/peds. Hence, I won't worry about it."
So, you'll need to convince the Med/Peds PD that you really are interested in only IM. You can certainly say "Med/Peds is truly my passion. It's what I want to do. But, I know that spots are limited and (based upon the number of interviews I have received) I am, on paper, not the most competitive candidate. Hence, I need to be realistic. I might not match in Med/Peds, and if not I want to match in IM instead. And since this place is so good, I'm applying to both programs." Of course, they would be likely to tell the IM PD this...
So, the bottom line is you need to be ready to explain why you applied to both programs, and which one you are most interested in. It's a totally fair question, and you can't tell each program that they are #1 (i.e. you'll need to choose whether IM or Med/Peds is your "passion" and stick with it, and tell the "other" PD that they are #2). This could be uncomfortable, but as others have mentioned it's unavoidable -- even if you cancel now, I'd ask you at your interview "So, I see you cancelled your interview with the Med/Peds program. Why did you do that? Are you no longer interested in Med/Peds?" ... and you really should tell the truth.