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Open Multiple Gmail Mailbox's at Once

Open a Gmail account (either your or your Personal account). 
In the upper right hand corner click the face icon (or it has your personal image).

Select the email address you would like to sign into.

Click "Sign in to another account" at the bottom of the list.

The Inbox will open in a new tab.  

If the Account you are attempting to access is not listed click "Add Account"

Enter the account address
Enter the account password

Click "Sign In" 

The new account will open and you will have access as prescribed by the owner of the mailbox.