Hey Harsh! Unfortunately it is not possible. You can not do so as to be eligible for JEE Advance 2020, a candidate need to appear in JEE Mains 2020 without qualifying JEE Main a candidate can not sit in JEE Advanced. So prepare for the exam as JEE Mains is a qualifying exam for JEE Advance.

No. You will have to give both the exams in a particular year. To avail the JEE Advanced, you will have to avail the clearance from JEE Mains. Without it, you won't be getting to go to the Advanced level. The eligibility for 2019 expires in the next year and you thereby have to appear for the Mains from the start.