You should plan your study in the way that you prepare for both the exams simultaneously. You should be aware of complete syllabus (JEE+Boards) and you should make your own time table. So, that you can complete all the syllabus on time and you get proper time for revision. You should set your targets daily and weekly.
You should focus on to clear your concepts and to build your fundamentals strong. You can easily score good marks if your basics is strong. You can take help of NCERT books for your preparation.
You should be aware of the exam pattern. Go through previous year papers to know the pattern of the exam and to know the level of questions.
Revision plays a good role in scoring good marks. Make sure you give enough time to revise your whole syllabus give importance to the topics which carries more weight age in the exam. You can also solve MCQs related to them.
You should also solve complete mock test series. Give importance to time management as both the exams are lengthy. So, your speed matters a lot. You can also solve timing test to increase your speed. Mock test series will also help you to analyze your preparation level and to know your weak and strong areas. So, you can work upon them.

All the Best for your preparation!!