If you have analog DID's you can delete all four digits and add the digits for an adjunct extension.
The adjunct extension can have VM cover applied and go to the same auto attendant as the main lines once it hits your voicemail
If you really wanted to get snarky, you could create another adjunct extension with Remote call forwarding. Assign the adjunct an authorization code. Then from any extension you can pick up dial tone, press #80, the authorization code for the adjunct extension, *33, 91xxxxxx#
Now when the caller hits the AA, they can press 1 to transfer the the adjunct which forwards to the other line.
Whats nice about routing calls directly to AA, is that you can uplug your phones while you move furniture