Outlook 2010 is able to open up to 15 Exchange mailboxes concurrently, not all of which have to belong to the same exchange organization. By default, Outlook imposes a limit of four mailboxes. However, you could increase the limit for concurrent open mailboxes to 15 by updating the value:

HKCU\Software\Microsoft\Exchange\maxNumExchange

Add-MailboxPermission -Identity 'Q1' -User ExAdmin' -AccessRights 'FullAccess'

Add-MailboxPermission -Identity 'Q2' -User ExAdmin' -AccessRights 'FullAccess'

Add-MailboxPermission -ID Telus1 -User "ExAdmin" -AccessRights 'FullAccess'

When you open ExAdmin mailbox in Outlook, it will automatically open the Q1,Q2 and Telus1 mailboxes.