Not sure there is a way to have 2 accounts using the same data files, the way I keep several e-mail accounts on several computers in sync is to leave messages on the isp server and only delete messages from the server when they are in the delete folder of Outlook. thus from which ever account reads the e-mails it always gets all e-mails that havent been deleted. Contacts and addresses can be imported from the original account and then should keep up to date when new e-mails come in. (not forgeting that they have to be exported from the original account initially)
I think the default setting is deleting e-mails from the isp server when they are read and there is a tick box somewhere in tools options ( or accounts) that allows the above to work.
Good luck
Rickity