If you have two domains that are identical (ie. company.com and you want to add company.net) then you can simply reroute company.net to company.com by doing as zbnet suggested in point 2, except set the new domain as Rerouted to company.com
That way, you don't need to add new e-mail addresses to each account, a mailbox with myname@company.com will receive e-mails addressed to both myname@company.com and myname@company.net