If you right-click on one of the tabs, you can choose to open it in a new window. You could then resize/arrange/tile/cascade the windows to be able to see them both at the same time.
You can only right click and select open in new window on a single document in 8 and 8.5, not a view. You could open webmail and select another person’s calendar.
Notes 8.5 has the ability to use calendar overlays, which will show multiple calendars entries on the same window. You can color code different calendar sources for distinction. Calendars can be other notes users, notes teamroom calendars, application calendars, Google calendar, or an iCalendar feed.
You could also create a group calendar for the most commonly used or department calendars
The alternatives mentioned above are true, but the limitation about not being able to open a new window in 8.5 is not. I just did it.