Hello aspirant.

If you don't qualify JEE mains examination  in January ,but you qualify in JEE mains examination in April you are eligible to write JEE ADVANCED examination. NTA (National testing academy) will consider best of two exams, it will ignore lower score .

For more information refer

https://www.google.com/amp/s/engineering.careers360.com/articles/which-jee-main-score-will-nta-use-for-common-rank-list/amp

All the best .

Hello! Hope you are doing well. Of course, you will be allowed to attempt JEE Advanced even if you qualify in either of the exams conducted in january or april.From this year, you will be given two chances and you can attempt either one of the exams or both and the best score of both the exams will be considered. So, even you did badly in the first attempt you still have a chance and can do well in the second attempt and similarly, if you are qualified in only one of the exams, then you can still attempt the advanced exam. And you still have a chance, so all the best and do well.The eligibility criteria for JEE Advanced is in the following link:

"https://engineering.careers360.com/articles/jee-advanced-eligibility-criteria".

Hope this information is useful to you. Good luck!