MX Records - go to two servers?

There's no way you can do this via the MX records within a single domain name--DNS is always going to point emails destined to [email protected] and [email protected] to the server that answers at the highest-priority MX record. (In other words, the second -highest priority MX record is not used unless there was no response at the first IP address. If you connect to the first but it rejects the email as a bad address, the sending client is not going to re-try sending to the next MX record.)

However, you could do this with a subdomain. For example, your main email server (and MX record) would respond for all emails destined to acme.example, but your Exchange server would only respond to emails like [email protected]. And one trick you could do to make this a little easier would be to have the main server except emails for [email protected] and forward them all to [email protected]. (You would want to configure Charlie's email client with a "Respond-To" address of [email protected], to make it less confusing for the senders.)

Also, there's a big caveat with the above trick: since Exchange is so tied into the Active Directory domain, it's not easy to move users to subdomains or change the subdomain that the Exchange server is answering to... so it may end up easier for you just to convert everyone off the old server to the new Exchange server all at once.