I'm not sure it would be possible to link both accounts to each other. If you wanted to link only one account to the other you could use Message Retrieval
and SMTP Accounts
. That would allow you to go to Mailbox A and view and respond to messages from Mailbox B. However, Message Retrieval wouldn't work for linking both accounts to each other. The reason is if you have them linked with Message Retrieval, every time you get a message in Mailbox A it would also be received in Mailbox B. When received in Mailbox B, it would be sent to Mailbox A. So one email would create an endless loop between the accounts.
I tried thinking of a way to set up one account with Message Retrieval and then use content filters / forwarding / aliases for the other account, but anything that I come up with would result in that endless looping between the two accounts. If you try something out and it works, if you could, follow up here with what you did. I'm interested in knowing if a workaround would be possible.