Open another mailbox in OWA
Open OWA by going to Office365.com and enter your email address and password.
Make sure you are on the Outlook Web Application.
If your not then click the corner option and select Mail
Once OWA (Outlook web access) is opened then click on your Profile in the right side of the screen.



Type the name of the mailbox you want to open and click open (this will open in a new tab)