Hey Tejas, you need not to worry about this as Jee preparation is not a troublesome task if you are doing it willingly and on a regular basis.

Usually, some topics in class 12th are the continuation of those in class 11th so it will be easy for you to keep the preparations for 11th up.

You can also take a class 11th coaching for a particular subject that you can't easily understand on your own.

In most of the school's syllabus for class 12th if completed by the October and only pre Boards are left, so you can do the preparation for your remaining 11th syllabus in the last two months and perform well in your jee mains paper.

Hope you will understand.

For further query regarding this please ask in comments.