How can I use the same sender address for two Exchange mailboxes?

Exchange (any version) alone does not support the sharing of email addresses between mailboxes or users but the ChangeSender add-on makes this possible:

First you need to install the ChangeSender server components on the Exchange server and the Change Server Outlook Add-on on the clients.Download ChangeSender

On the server use the "Configure ChangeSender and Client Restrictions" Tool to allow the users to use any addresses, not just the email addresses from their ActiveDirectory entry.

On the clients open up outlook and a new email, then click on the properties icon next to the "send as" selection box to open up the ChangeSender configuration. Here you can add the address to be used as one possible selection or even as the default one.

There is no limit in the number of users you can use the same email address for. You can let all members of your sales team send out emails as sales@yourcompany.com for example or let everybody answer emails to info@yourdomain.com without revealing their personal email address

In order to distribute emails sent to a shared email address to a group of users you can create a distribution group in Exchange, give the shared email address to that group and add all members of the group.