Second address is a synonym for another address
If the second address needs to be a synonym for another address, you make use of a alias. An example is firstname.lastname@example.org, that could be a alias for email@example.com. This way, e-mails that are sent to firstname.lastname@example.org will arrive at email@example.com.
Ask the administrator of your e-mailserver to set an alias.
Second address needs to redirect to another address
A 'redirect,' or 'Forwarding' address is a e-mail address that doesn't receive the e-mail itself, but sends the e-mail directly towards another address. This can be an address of another domain.
Ask the administrator of your e-mailserver to set a redirect
E-mail that arrives on this address will be send to your own address.