Sure it is, just takes doing 4 residencies or fellowships. That would be psych, pediatrics, child psych (a subspecialty of psych, not peds), and then pedi cards, pedi EM or PICU. I think triple boarding is still around (not sure) which would simplify the first three in terms of timing, but I can't see much of a way around then doing the 4th one as a fellowship.
In addition to going broke from board exams, you'd not really have much chance to practice all of these. On the other hand, a PICU doc or pedi cardiologist who was a board certified pysch and child psych would definitely be an advantage.
Also, I'm not at all sure I agree that child pysch is particularly depressing compared to pedi critical care specialties or some other pedi specialties such as neuro. The hours are usually a bit better than the pedi subspecialties, but, I don't think most folks go into any of those fields focused primarily on the hours.