In the users AD information, there is the Email Addresses tab, find the users you want to be in the other domain and change it from @abc.com to @xyz.com. Your DNS for both domains should point to the same server, but when the emails hit the AD server is sees Bob@ABC.com and directs the mail to his mailbox and Jim@XYZ.com and directs his there. When they send, it should work the same way.

Again, I’m not an exchange expert and haven’t actually done this in real world situations, our exchange usage differs from most companies in that we use exchange ONLY for calendar sharing, not for an actual email server…