Yes.  You can have the Joomla Administrative Interface (aka Back End) open in more than one browser tab.

Once you are logged into the Back End, the browser will consider you to be logged in for all of the tabs open in the same browser session.  If for some reason you want to have different browsers open, for example Chrome and Firefox, you will need to log in to each browser separately.

In fact, this can be a great time saver, if you are careful. You can have the Menu Manager open in one tab, the Module Manager open in another and the Article Manager open in a third.  This save the time of having to navigate through multiple pull down menus for different tasks.  

The caution that you need to take is not to be in the same item at the same time.  If you click on Save in one tab, but you were adding content or making changes in a separate tab, you risk losing those changes.  The same thing goes for people using the same Login on different computers at the same time.