These are just guesses, I've got 5 accounts and I'm not going to try these out!
Can you make your 'store folder', ie where you store all your messages, a folder in the shared documents directory so that you both access the same inbox etc?
Or have a common user account, that you can both access, just for email.
Or you could choose to 'leave a copy on the server' (under Tools, Accounts, Mail, Properties and the Advanced tab) that way you could both download the same messages - but I guess you'll download your own mail again next time.