No, 
11th is more important than 12th because in 11th you learn basics and basics is more important for IIT specially for mathematics
You can take importantance of 11th as 60 percent and rest of all as 40 percent. 
						

							
	
Practice previous year questions many times..
	
Appear for a test series and analyse it properly (Giving 2–3 hours)
	
Revise your notes multiple times till they enter your soul :)
	
Find your strong points and chapters, work on them..
	
Solve other people’s doubts. It makes your own concepts very clear
						

							
11th and 12th, both are equally important. If your concepts are clear then you should focus on 2 things- 1. learn formulae, 2. solve previous year questions.
Important topics 
11th – geometry(because of it’s weightage)
          algebra(as it includes basics and a very important chapter, complex no.)
12th – calculus(because of it’s weightage, it isn’t difficult, if once you’ve learnt all                       formulae)
           vector and 3-D(it is not only important for maths but is sometimes also used for               solving physics questions)
           a bit of algebra which is also there(matrices and determinants, which obviously               no one will want to skip)
Trigonometry is also important as sometimes questions are asked from this unit and many times it’s formulae and concepts are used in solving other questions.
 
Yes, it is almost the complete syllabus. So it will be better for you to arrange the chapters in your way, from weak to strong or vice-versa :)