If we prepare well enough for JEE Mains does it help our JEE Advanced Preparation,

or is the converse true?

JEE Advanced prerparation helping our JEE Mains Preparation?

Both need focussed hardwork, you should see previous year questions and the current syllabi then you will easily figure out the difference.In jeemains you are required to know the concepts used in the question and for jee advance you need to have clarity in each and every concept apart from average IQ(to figure out the correct approach).So a thorough study of topics is the basic requirement then you need to give extra time to each topic for appearing in a particular exams where you focus on the syllabus,trends and the level of questions asked in that exam.