Hey don't get confused, just do your JEE syllabus first. Then from February onwards (1 month before the exams)
devote your time in cbse only. See, in PCM, while preparing for JEE, 85-90% of CBSE syllabus is covered
simultaneously. So in the last month complete the left portion of PCM (which was not covered earlier and is only in
CBSE, not in JEE syllabus) and the other two subjects (Eng. and P.E. etc ). But till then no need to panic for the
boards. Study 6-7 hrs daily for JEE. And yes you must read the NCERT books (PCM) while preparing for JEE because they are required for
developing basics. By doing this you can easily manage your entrance as well as CBSE preparation.
Best of luck! :-)