hey ajay 

i will suggest you should focus on single competitive exam and get impressive score in it for great career ahead. for NEET you have to prepare biology thoroughly and for NDA you have to prepare maths thoroughly. if you will prepare both simulataneously then you might get average score in both which will be of no use to get good colleges. if you are highly intelligent and determined that you can focus on both subjects equually then only you should prepare for both. otherwise choose single career and prepare it thoroughly. 

thankyou. goodluck.