My suggestion was to use two (or more, if necessary) different browsers. They have their own sessions, so you can log in simultaneously with both users.
As an example, you can use Internet Explorer for one and Mozilla Firefox for another user.
Pretty simple!
December 16, 2016 update:
Currently, when browsers have entered private sessions (e.g. Incognito for Chrome), you can simply open this closed session and log in with it. He will not use storage mechanisms associated with the main browser window.
Although, in Chrome, at least - opening two windows incognito will not force you to get a total of 3 sessions. Incognito, they all live with a "solitary" session, so to speak. I do not know about Firefox and others.
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