Multiple Domains - 1 AD user 2 seperate mailboxes

Currently we have a Windows 2008R2 AD domain AAA.LOCAL with Exchange 2010 hosted domain for BBB.COM fully functional with OWA, AcitveSync etc.

I need to add additional company domain CCC.COM without creating a new separate AD domain and users.

How do I achieve the following or please recommend another way:

1. User AAA.LOCAL has separate mailboxes and can send and receive from BBB.COM and CCC.COM.
2. Have separated access to OWA MAIL.BBB.COM and MAIL.CCC.COM

I thought about installing a 2^nd exchange server for the new domain only.

I’ve read a lot of posts but not sure which way to go,

Can you please provide detailed configuration?