You could set up the authentication by simply adding SSH keys to the two machines. On the cPanel machine, go to WHM > Manage SSH Keys and click "Generate a New Key" option and keep the defaults. The new key will be called id_dsa. Back in Manage SSH Keys area, click on "View/Download Key" and copy the public key there. Go to the machine where you will be scping the files and if it has WHM, go to Manage SSH Keys area again, select Import Key, paste the key into the public box and save it. After saving it, select "Manage Authorization" back in the main Manage SSH keys area and authorize the key.
If you do not have cPanel on the machine you will be scping the file, then ssh to the machine and cd to /root/.ssh then save the key in authorized_keys file from the previously saved contents. Also, place a copy of the public key as root@fullhostname with the contents copied previously.
Test it is working by going back to the cPanel server and trying to ssh to the one you'll be using to scp the files. If it doesn't ask for a password this time, it's working.
Now, the point of having SSH keys would be that you won't need to use a password any longer from one machine to the other. At that point, you can have a cron scp the backups without it even requesting a password.