See, you can give exams from two separate boards only under the condition that the exams given must be in two separate sessions, like if you have given the CBSE board exam in 2020, then you can give the NIOS board exam only in 2021, and not in 2020. So, though you CAN go for PCM in NIOS, but you CANNOT complete the certification course along with your normal CBSE exams.
Hope I have answered your question. All the best!
Regular exam updates, QnA, Predictors, College Applications & E-books now on your Mobile