Multiple domains in Exchange Server 5.5

Hi,

I have an exchange server, with domain name (for example)xyz.com,
now i like to add another domain name, to the same exchange server (for example) abc.net.

I have 20 users, i need all the 20 users, should be able to recieve mails in bothe-mail addresses and
They should be also able to send from any one of the two
e-mail address, as they prefer.
(Users need both e-mail address for sending and recieving)

Can you please advise me how to achieve this?

Thanks in Advance.

Regards
VIJAY