You should really pick one. You can certainly apply for both, but in order to have reasonable chances of matching, you would have to go to way more interviews than most other applicants (especially if they're in person), and your program may not allow that.
Once you pick a specialty fellowship, these days its hard to practice good medicine outside of your trained specialty because things change so much and you need to hone your skills and judgment after fellowship in your said field. Diluting what you do will just make your skillset less optimal than your colleagues.
If you want to do it all, you should just stick with comprehensive, and seek out opportunities as a senior to get more hands-on experience in these fields,. That way you can do some of both fields and refer out if the patients have more complex issues.
