This is similar to how I have things set up on my two main computers (I have two offices, truth be told).

The real answer to this is simple: IMAP. Using the Internet Message Access Protocol you can have all your messages live on the mail server but be accessed by both of your mail clients in different locations. Delete the message in one mailer and it automatically is deleted from the other client too because it was never on either computer in the first place.

I'm not a big fan of IMAP, however, because I don't like having my messages live on the server rather than on my own local computer where it's very easy to do backups, burn archival copies to a DVD-ROM, search through the messages with Mac or PC-based tools, and more. Further, I worry that if the server crashes, I now lose my email archive, which would be a significant problem.

Finally, IMAP takes lots of resources on the server - my email archives are in the gigabytes - and are also accessible to any other user, administrator or hacker who might sneak onto the server, versus my own computers where I know exactly how it's all protected and secured.

So how do I have mine set up? I use the common POP (Post Office Protocol) and on my "main" computer I download messages as they arrive. On the secondary computer, I get a copy of the messages instead (that is, I have selected "leave a copy on the server" as part of the POP account configuration. Actually, I also have "delete from server when deleted from local mailbox" too, so I can at least trim down the daily tsunami a bit)

I'm the first to admit that this is a mediocre solution because I then have to actually remember to turn off my main computer when I'm not in the office (or it grabs incoming email without me ever being able to see it on the secondary computer) and I also have to transfer pending messages onto the secondary machine so I can continue fielding them.

Those are the only two meaningful solutions I know about, though I have heard from some folk that they use tools like VNC and other "virtual desktop" tools to actually hook one computer directly up to the app running on the other computer: then it'd be like the email app itself bounced onto the appropriate screen without any fuss. That'd be easy to do with the X Window System, but PC and Mac users don't quite have that level of flexibility, plus there are security issues with having to breech the firewall at work, etc.

So I'm afraid I don't have any great solution for you, but perhaps someone else reading this can offer some novel ideas for having two email programs in two physical locations stay in sync on a single mailbox?