For Google Chrome Users
If you are using chrome then follow there steps:-
1. First open your first Facebook account and login with your ID and password. Now your one account is active, time to open second account on the same browser.
2. Go to the Facebook logo on the left top of your Facebook Page, right click and click on Open link in Incognito window (as u see in image).
A new Facebook login page will open in a new window of same browser, put the Login details of your another account and enjoy the access of double accounts on the same browser.
Alternatively, Directly do it by pressing Ctlr+shift+N to open an Incognito window and browser multiple accounts.
What is Incognito Window
Incognito windows does not remembers your history about the pages you visit. neither it keeps any type of page cache.Pages you view in this window won’t appear in the browser history or search history, websites can not leave other traces, like cookies on our browser.