Dear aspirant,

Personally, if I consider my case I myself had not studied chemistry in class 11 and I can also say that I haven't studied nicely in class 12 too(particularly in chemistry). But as your exams are near so you  have to manage your preparation by any means because you have to score atleast a decent in class 12th.

Secondly, if you are planning to give engineering entrances then you have to strengthen your class 11th chemistry because in all entrance exams , questions are asked from class 11th and 12th syllabus and in jee particularly almost 50-50 weightage is given to class 11th and 12th.

So,for exams of 12th you can start with your preparation with your 12th syllabus only ,no need to go for 11th but for engineering you need both.