Hey aspirant,

As a first step in your Jee preparation you should be clear with the fundamentals. So it is better to do the Jee main part at first and then later solve Jee advanced questions. If you are thorough with the topics then you can give test for both parallelly. Since you have not specified your year of examination it is difficult to answer the query. Even though the syllabus for both the exam are somewhat similar the way of attending the paper varies. The best advice is to only go for advanced if you are done with your mains part. Also you should  concentrate on mains at least two weeks before Jee mains if you are currently focusing on Jee advance and you can use that time to give a lot of mock tests. And it will be better if you can give both papers in alternative manner.