Dear student,

If you have Maths as main subject and Biology as additional subject, then don't be worry. Your both exams will be conducted with your other class students. CBSE releases the datesheet for class 12th, and it includes dates for each paper. If you have biology as additional subject, then you will give biology paper on the day, when students having biology as their main subject were giving their biology paper. There is only one datesheet for CBSE papers.

Hope it helps.