Like Tim mentioned, the only way you are going to get certified in both, at the present time, is by doing a dual EM/IM plus the cardio fellowship. Since you are still an undergrad, it is an issue that you can consider with a lot of leisure.
I would say that you need to look at the usefulness of doing such an unusual program. Being trained and board certified in EM will give you all the expertise you need to treat cardiac emergencies in the ER. If you are working as a ER physician, you will not have the luxury of acting as a cardiology consultant for your ER patients. Why? Because you still have the LOL with pneumonia in curtain 3, the kid with the broken ankle in curtain 4, the r/o ectopic in curtain 7, and so on. It is just not the job you are supposed to be doing while acting as ER physician. Nobody keeps you from taking as many cardiology rotations as you can fit into your schedule and be an ER physician who is a real whiz at cardio cases, but you don't need to do a IM residency and a cardiology fellowship for that.
By the same token, if you are a cardiologist who is also board certified in EM, it doesn't mean that you can just waltz down to the ER and take over the cardiac cases.
The only reason I would see to go the EM/IM + cardio fellowship is if you just loved EM and cardio so much that you really wanted to be a cardiologist and still moonlight as a ER doc (not very realistic either, if you consider the schedule cardiologists keep) or if you are interested in clinical research involving emergency cardiac protocols (and even at that, IM + cardio fellowship should suffice).