If you want a particular E-Mail address to mail to multiple inboxes (a forward to two or more addresses), just setup a forwarder entry for each address in your cPanel. You can do it one by one, or do all the emails separated by a comma i.e. email@example.com, firstname.lastname@example.org.
Ultimately, you would end up with entries such as those below.
email@example.com -> firstname.lastname@example.org
email@example.com -> firstname.lastname@example.org
email@example.com -> firstname.lastname@example.org
This is done by going to the Mail Section within cPanel and clicking on Forwarders.