I'm going to be very honest with you here . . . one thing us pulm guys detest are the cardio fan boys who coudln't get in and now want to do pulm/cc, because we're "ok enough to do". So if you're going to make a play, apply to both, which will mean more money and more interviews (if your program will allow it), and don't apply to both at the same institution, ever. You will need two separate personal statements and letters of rec from from different pools of physicians - make sure you PD knows you are apply to two different places and to keep his letter non-specific (ie. instead of Dr. Handsome88 would make an excellent cardiology fellows, he says, Dr. Handsome88 would make an excellent fellow for any training program . . . whatever). If all of your research is in cardio, you will need to explain that in your PS and be prepared to discuss why you are no longer interested in cardio and just LOVE pulmonary now - make up an epiphany story. Make sure you LOVE pulm more than critical care.
Alternatively, you could go cards or bust with the next application cycle, and then apply to just straight critical care after if you don't match. IM critical care fellowship is kind of a no-man's land of wanderers and they are much more understanding and sympathetic (and need their spots filled).
Good luck.