In OWA 2010, it’s impossible to open multiple user mailboxes.
OWA 2010 will allow for only one user at a time per session. No matter what you use new tabs, new windows – you get an error message:
“You may have already opened your mailbox on a different browser tab. If you feel you've received this message in error, try refreshing your browser.”
To open multiple mailbox then on the browser click File> New Session.
Since most of the users use desktop shortcuts or predefined urls in the favorites tab, the smarter solution would be to create these shortcuts with the following argument:
"C:\Program Files\Internet Explorer\iexplore.exe" -noframemerging https://Webmail Address>/owa/<user's email address>
This will allow you to open multiple windows with multiple users mailbox.
Unfortunatelly, it does not work in tabbed browsing.