Hi aspirant 

thanks for asking your query as there are certain chapters in 12th that is completely autonomous to the 11th topic like electromagnetism in physics ,physical chemistry of 12th etc such topics could help you to score better in jee ,certain topics are also there which could help you but it is necessary to prepare both the years .

Hey Ishan! You can clear JEE Mains even with only 10 + 2 syllabus but by ignoring or not studying 11th standard syllabus you are making it difficult for you because there are some topic from 11th standard syllabus which are easy and can help you to score marks. So according to me you should go for both the class. You can also give Careers360 Talent Search Exam (CTSE) is an All India Talent search exam exclusively for the JEE aspirants. To be held on September 8, the Careers360 Talent Search Examination which  is a computer based test for class 11 and 12 students who will be appearing for JEE Main 2020/2021.

https://learn.careers360.com/careers360-talent-search-exam/?icn=ctse_scholarship_exam&ici=JEE_Main_text-mapping