Preparing for both NEET and JEE together is possible, but honestly, very tough.
The syllabus is huge. NEET needs strong Biology, and JEE needs strong Math.
Both exams have very different styles and expectations.
Only students with very strong basics and high discipline can manage both well.

If you are an average student, it’s better to choose one exam and focus fully on it. These are my honest advise to you, you can prepare for one exam and crack that exam, the future opportunities are many in JEE and NEET.