I don't think there is a way to automatically copy all your messages to more than one computer but there are two ways to accomplish almost the same thing.
1) If you use POP3, have one or both PCs set to "leave mail on server" so that you can see your mail on both but download and store it only on the computer that does not "leave mail on server".
2) If you use IMAP you are actually always viewing your mail on the server no matter which computer you use to access it.
If you're not trying to "sync" continuously but just want to export from one and import to the other, the export/import features will do that. In fact the default choice is "All Folders" but it makes more sense to choose "selected folders" and "control-click" the accounts and folders you want.