This is an old revision of the document!
There are various reasons that a user might need to manage two Exchange mailboxes.
This allows an Outlook or OWA user to log in with only the 'first mailbox' credentials.
Modify the second mailbox permissions so the first mailbox (account) has Full Access and SendAs permissions:
Exchange Admin Center → Recipients → Mailboxes → <mailbox> → Edit
Mailbox Delegation → SendAs → Add the First Account
Mailbox Delegation → Full Access → Add the First Account
The