In return for the joy that reading this thread has brought me, and with the hopes that it will never die, I will continue on with further serious concerns about your plan.
Let's say 6 years for MD/PhD, 15 years for your combined residencies/fellowships/subfellowships (+/-, we don't really know since it's never been done), you're done at age what, 43? I can tell you that even after congenital cardiac surgery fellowship, most of them go and become junior attendings to gain more experience before becoming primary.
So now you're in your 40s, and you're assisting other surgeons in 2 fields. And it's going to take longer to gain experience because you're getting it at half the rate that other people who focus on one specialty are getting it. How long until you're completely independent? 5, 7 years? So now you're finally independent at age 50? Then you have maybe 10 years before your skills start deteriorating and you start shifting into a supervisory role?
What's the benefit to your patients? To your department? There isn't any. Nobody is saying, "Man, I wish our amazing skull base surgeon could have less experience doing skull base surgery so he can be a mediocre congenital cardiac surgeon in addition." Literally the only reason to do this would be an ego stroke for you. I promise you, if you become a top-shelf subspecialized neurosurgeon or congenital cardiac surgeon, there will be plenty enough ego-stroking as is...