It is definitely possible. You've got to be very good at managing ur time & have a lot of determination. And be smart, but if you're planning to double major, u probably r :)
I do not know if you're planning on attending a film school to further the directing side of ur career, but in addition to med school, film schools r usually two years & r very expensive & competitive.
I've heard that directing major films is a HUGE undertaking that requires a lot of time & devotion, but if you've already established a stable cardiology practice & can arrange periods of time to focus more on directing & less on that, then u should be fine. It is going to take a while to reach that point, though.
Prepare yourself for like 15 years of school! I know cardiology requires at least a 4 year residency, but it will be a very rewarding career.