There are common subjects you can study together, but the NBDE asks the questions in a superficial level, enough to have an idea what it is, while the NMBE CBSE makes sure you understand why you picked that answer. I would suggest getting the NBDE part 1 out of the way first, and you can probably pass it by doing the dental decks once or twice. Also, it's not scored, so as long as you passed, you're all good. I wouldn't spend more than 2 weeks studying for it. You can take a practice "Crack the NBDE" to see if you can score comfortable above a 75 to pass.
The NMBE CBSE is about 18 times harder, and requires you to really put in a lot of effort to obtain a good score. I would suggest at the minimum 2 months dedicated to studying for it.
Sample NBDE part 1 question:
Which substance, synthesized in the endothelium, is responsible for vasodilation of smooth muscle?
a) Substance P
b) NO
c) Adenine
d) Bradykinin
Sample NBME CBSE question:
A 46 year-old man is being evaluated for occasional retrosternal chest pain. He says that it develops with moderate exertion but sometimes occurs when resting. Coronary angiography shows mild luminal irregularities but no significant obstructions. Acetylcholine infusion during the procedure results in dilation of his epicardial coronary vessels. A reaction involving which of the following amino acids is most likely responsible for the observed effect.
a) Tryptophan
b) Aspartate
c) Arginine
d) Histidine
Both questions are asking about the same thing. But one requires more critical thinking to reach the answer than the other.