First, you should think that every year there are a lot of ISC candidates who qualify NEET thus you must be sure that its possible for ISC candidate to score well in both. Now the right strategy for cracking the examination is to find the portions which are common in both NEET and ISC. Do those first. This will help you to gain confidence as well as you will not have to invest a lot of time separately. See the concepts are the same so you can study the same concept at a time and then invest time for subjective problem solving(boards) and MCQ solving for NEET. This way you will cover up the entire syllabus along with your boards. Also the matter of fact is 70-80% of the ISC Syllabus is the same as of NEET. So you have to read the 11th portion in excess but I would say give more priority to the portion of syllabus which is based on 12th. If you complete that portion then you will feel relieved and can focus more on the 11th portion. So you have invest 70% of your study time on the concepts of 12th syllabus of NEET and 30% of your time on the 11th portion.

To know more about the NEET preparation tips visit the link below:

https://medicine.careers360.com/articles/neet-preparation-tips

I hope this answer helps. All the very best for your future endeavors!