Why not just do a 2 year critical care only fellowship?
This is a good question. Alternatively, if you do cards, you can tack on a single CCM year and be boarded in both.
FWIW, our program has a CCM only track and a lot of the fellows finishing up are finding it hard to get jobs in the area, or in other major metro areas (at least on the West Coast). Everybody wants someone who can not only set vents, put in lines and choose pressors but also do bronchs and get difficult airways.
IMHO, if you want to stay in academics, it would be tough to do with CCM only unless you've got a funded research project to make you marketable.