Dear Gupta
Once you have filled application by providing the Aadhar number you cannot make any chances regarding the selection of papers. As you have given paper 1 you cannot use the same details and register again for the same details so you can go through the document about JEE related queries and I'd necessary can call the helpline and can get a clear picture.
All the best.

Hello there,

Your question seems to be unclear. Please ask your question in proper way. Because it is unclear that whether you want to ask that you have registered in the first time for paper 1 then you want to register for the paper 2 separately or you only want to register for the paper 2 only. so this thing is unclear in your question
However in this type of situation you can contact on the JEE mains helpline numbers which are easily available on their website:

Jeemain.nic.in

Good luck

Joint Entrance Examination (Main)

Application | Eligibility | Result | Preparation Tips | Question Paper | Admit Card | Answer Key | Accepting Colleges

Get Updates Brochure