Ok, I just noticed that a lot of people were asking this question and no one gave a ‘tested answer’.
It took only two mins for me to test this out and the answer is YES, two people can be logged into same Facebook account from two different computers, either on lan or anywhere in the world !
Now after this I tried to figure out if, facebook had any such feature that would allow me to look at my past login session details (yes, something like the gmail feature, which allows you to check who had logged in recently 
), and I could not find anything 
.
), and I could not find anything .
So, to conclude, two different people can log into the same account from different computer on facebook and this goes unnoticed, as there is no way to figure out who logged in and from where!
Latest update: All those who were logging to someone else’s account at the same time when they were logged in may not be able to do so. Facebook has improved their security, as a result of which users can know who else is logging into their FaceBook account and from which computer.