Exchange Server 2000 & Multiple e-mail domains

I have a domain running its own domain e-mail (D1). I have a second domain (D2)which shared the same users and requires each user to have a D2 alternate e-mail address to receive D2 mail and reply to D2 mail. I have it where users can receive the mail 100% but they can not reply with D2 mail address. I need to resolve this if its even possible. Each user needs the ability to receive and send with their D2 address. Step by step is best since I’ve beaten the house and got only half way there.