Hello Anushka

If you have few chapter or syllabus remaining for class xi then you must first complete class xi syllabus because if you leave any topic without understanding,  you won't bd able to cover it in class xii.

Now for class xii whatever you have block logs you may complete them side by side , for that yoh will have yo ho bit fast to balance your class xi and class xii .

Jee main exam paper is based on both class xi and class xii , also we learn basic things in class xi so both are important .