I am relatively new to PHP and Joomla. Basically I am building a website which allows registered users to login and use the online services that we provide.

I would like to know if its possible to achieve the following:

1)Allow only one user at a time to access certain pages in the website
For example, only John can access page X , Tim can access page Y concurrently. No two users should be allowed to access page X or Y at the same time.

Im not sure if im heading towards the right direction, but i was thinking of adding an additional column in the database, that will be marked whenever a user enters that particular page.

Your help is much appreciated :D

Sounds like a bad idea. What if John quits his browser after opening page X. He's locked Tim out for ever.
Then you'd have to run a timer, Tim can look at X n seconds after John? John can perform tasks on X for only n seconds?
HTML is not designed to do this kind of thing.
The question is why, what are you trying to do and maybe there's a better way.