We started using SmarterMail on version 14, I then upgraded to 15. On these versions I could not be logged into 2 different mailboxes at the same time. But, you can get around this by using different browsers or same browser with different sessions. However, after upgrading to SmarterMail v16 I was allowed multiple logins on the same browser with different tabs.
This is very useful to me as I use it to be logged into my mailbox along with the admin mailbox.
Hope this helped.