Internet Mail & Exchange 5.5

We changed our domain name from company1.com to company2.com. We host our email on Exchange 5.5 using Roadrunner and 1 static IP. Management now wants all users to send/recieve their email as user@company2.com, but still recieve email from user@company1.com until all our customers are aware of the domain/email change.

Here are my questions:

1. Can we have two different MX records (mail.company1.com & mail.company2.com) point to the same IP address?

2. How do we set up Exchange 5.5 so that the user’s mailbox recieves email from both (mail.company1.com & mail.company2.com), but when sending email only mail.company2.com is used?

Thanks for the help!