How to merge 2 WordPress sites?

I have 2 WordPress sites, A and B. Each of them has users and posts. Now, I want to merge these sites. Points to be noted-

1. 
   

   A has 1000 users and 2000 posts. B has 500 users and 3000 posts.

   
   


2. 
   

   There are some identical users. Say, Rahim and John have accounts on both sites. Rahim wrote 3 posts on A and 2 posts on B. Again, John wrote 0 posts on A and 5 posts on B. On A, user_id of Rahim is 3, on B it's 2. Similarly, John has different user_id on 2 sites.

   
   

Am I clear? Now, how can I proceed? I want to keep all the posts to original authors and unify duplicate users.