How do I sign into a different email account without logging out of the other one I’m also using?

I am trying to be logged into 2 of my email accounts at the same time.  But when I try to sign into the 2nd one via 'Hotmail sign in' it just logs me into the same account that I am already logged into.  I do not want to log out of the account I am already logged into given previous problems with logging into it and I have urgent business I need to attend to that is already delayed.  I know it is possible to have 2 email accounts accessible and logged into on the screen at once as have seen it before, but unable to find how to do it now??????