With a POP account this will be difficult. You can ask OE to leave a message on the server so that you can download a message more than once but the Sent Items etc etc will not match.
Hotmail accounts can do this. Forward your mail to a Hotmail account and then each OE looking at the Hotmail account will see the Inbox, Sent Items etc on the server and will always be the same. Any changes made by one PC will be updated on the other the next time its synchronised with Hotmail.