Go to /root on 1st server.
FreeNAS OS drive is mounted read-only, so mount it RW.
mount -o rw /
Generate an RSA key & leave the passphrase blank. You can use another supported algorithm if you wish.
ssh-keygen -t rsa
Display the public key.
Copy / paste the key to a text editor and remove any line breaks. The key should appear on 1 line. Leave the spaces intact (e.g. the space after "ssh-rsa").
Copy / paste the key to /root/.ssh/authorized_keys on the 2nd server.
Repeat the above process for the 2nd server. Create the key on the 2nd server, and copy/paste it to the 1st.
Test your new SSH keys by SSH-ing into the 2nd server from the 1st and vice-versa. The first time you'll get prompted to trust the key. Accept the prompt to add the host to known_hosts. Disconnect and SSH again. It should connect immediately without prompting for a password.
The keys have been swapped successfully, so you can now setup your rsync jobs.