There are two tasks for this:

1. Assign Full access permission to user B on User A’s mailbox

2. Give Send As/Receive As permission to User B on User A’s mailbox.

Full Access Permission:

Run the cmdlet:  Add-MailboxPermission “UserB” -User “UserA” -AccessRights FullAccess -InheritanceType all

Receive As/ Send As permission:

Add-ADPermission “UserB” -User “DomainUserA” -Extendedrights “Send As”

Add-ADPermission “UserB” -User UserA -Extendedrights “Receive-As”