Now this situation is very similar to mine , I can tell you something that might help you , it's your 12th , so I'm guessing that you are a regular guy who attends his school , first of all it depends on how many hours you study in the whole day , let's suppose 6 hours and by studying I mean practicing not doing homework and all , so let's suppose you study six hours a day, devote 4 hours to the twelfth syllabus and then two hours on 11th you can also do more for eg. You study 8 hours a day , so you should go with 6 hours with 12 and then 2 hours with 11 , now comes the most important part how to study while doing 12 also , well it's simple take a day to revise a chapter (11th) then next day do the questions related to that chapter and similarly you can do this with class 12 th syllabus also. And do remember don't postpone your task . Otherwise it becomes a habit and till the end of the session you will enjoy this . So be straight and concentrated . Hope it helped.