Share a Mail.app mailbox between multiple computers
I had a similar problem a few years ago and decided to move to a better email service. I chose Gmail, mainly because of the >10GB storage. 

You can migrate your locally stored messages via IMAP. Once you set up your Gmail account, add the IMAP server to Mail. Then drag and drop all the messages you want to migrate on the Gmail inbox. IMAP will do the rest. 

Adding the IMAP server to multiple devices will obviously keep them all in sync. 

In general, I would recommend this over using a local option. IMAP is a superb protocol and I don't see the advantage of trying to sync the local archive in any other way.