First create a user

 useradd data-sync 

then create the ssh key

ssh-keygen -t rsa -b 2048 

finally copy the key over to the other server.

  ssh-copy-id -i ~/.ssh/id_rsa.pub data-sync@other-server

finally try logging in on the other servers using ssh

You can also copy the key accross using

cat ~/.ssh/id_rsa.pub | ssh user@ssh-server-ip "cat >> ~/.ssh/authorized_keys"