There are some fields where you can practice multiple specialties (and be multi-boarded)
Examples include Internal Medicine-Pediatrics, Pulmonary-Critical Care Medicine-Sleep Medicine, Psychiatry-Sleep Medicine, Psychiatry-Neurology, etc. There are a few Cardiology-Critical Care trained doctors as well. It's doable, but long training ...and also finding t he right job that will let you do what you are trained to do. Downside ... It is a pain to maintain multiple board certifications (MOCs)
Disclaimer: I currently hold 2 separate board certifications (and going for more)