FYI, the match rate for derm is not that much better than the match rate for cardiology. Last year it was 47% for derm and 52% for Cardiology. However, if you look at the US Grad match rate, it was 55% for derm and 75% for cards. A bit better, but discouraging nonetheless.
Advice. If you want derm more than IM then go for it. You can apply to both derm and IM programs and rank all the derm programs above the IM programs so if you do not match derm, you won't have to scramble for anything. This will cost more money but may be worth it in the long run. Your best chance of getting cards is to go to a solid MD program, so scrambling would not be a good idea.
If you are interviewing for derm and they ask you why you are applying into IM as well, just tell them you really want to do derm and there are two ways to do it, derm and IM. Tell them you realize that derm is competitive and that you want to have a back up choice in the case you do not match into derm, a back up plan that will still give you a chance of doing it in the future. Derm programs know theyre difficult and they will not be surprised if you take the proper steps towards a back up plan. You do not have to tell them you want to be a cardiogist as a back up plan.