Hello,
EAMCET is conducted for admissions to Engineering, Agriculture, Medical undergraduate courses in AP and Telangana while JEE Mains is conducted for admission to B.Tech courses in NITs, IIITs and other engineering colleges.
The syllabus for both the exams is based on class XII science subjects i. e. Physics, Chemistry and Mathematics.
So, the syllabus is same for both the exams. However generally speaking the JEE Mains being a national level exam has higher difficulty of questions as compared to EAMCET.
So, it would be best to prepare for JEE Mains and your preparation for EAMCET would be done automatically. Just go through the previous year question papers of both to get a fair idea of the comparison between the two exams.

Follow this link for more information EAMCET
https://www.careers360.com/exams/ap-eamcet

Follow this link for more information JEE Mains
https://engineering.careers360.com/exams/jee-main

Good luck