Biology as an additional subject means not from 2 different boars but studying Physics, Chemistry, another subject with Biology as an additional subject at the same time. If you have completed your 12th from ISC Board, studying Biology as an additional subject from your State Board might not qualify you for NEET. This is because you should have studied Biology along with your other subjects and not as a single subject. In this case, unfortunately you might not be eligible for NEET.