With very few exceptions (I'm personally only aware of one exception), people who specialize after Med/Peds do either an adult or a peds subspecialty. You won't be unique in that regard ...
I wouldn't say "with very few exceptions" based purely on anecdotal evidence.
Roughly half of Med-Peds graduates go into primary care practice, and of this group, between 77-93% provide care for both adults and children.
About 18-25% of Med-Peds graduates go into fellowship training, and of those who do fellowships, at least 50% of them take care of both adults and kids.
Source: Freed et al. Internal medicine-pediatrics physicians: their care of children versus care of adults. Academic Medicine. 2005;80:858-64
As for combined fellowships - there are combined programs available. But if a formal combined fellowship isn't available, a graduate can always negotiate with fellowship directors to create their own combined program.
Combined cardiology fellowships are available at:
Baylor College of Medicine
Columbia-Presbyterian
Duke
Ohio State
University Hospitals of Cleveland/Rainbows Babies and Children
University of Pittsburgh
University of Rochester (Preventive Cardiology fellowship)
University of Texas Houston
Vanderbilt
For more information about Med-Peds, you can visit the National Med-Peds Residents' Association's website at
www.medpeds.org