Can I work in multiple documents at the same time?
Yes, and you can set your Preferences to have several different documents open in tabs or in separate browser windows. However, only one report from each document can be open at one time. If you try to open another report from the same document, it will replace the first one you opened from that document.
Your 60-minute BI4.x session is timed from LOGIN or your last activity in the application, not separately for each document.