How do I show a specific web page to a specific user

Suppose I have made 2 static web pages and hosted them using virtual host. Let the names be example.com/page1.html and example.com/page2.html.

Now user A has his own unique browser with a unique USER_AGENT string. I have to make sure that when the user A tries to access example.com from his browser, he will be shown example.com/page2.html.

If anyone else tries to access the URL, they should be shown example.com/page1

Please someone help me out with this problem.

UPDATE 1

This can be done inculcating the USER_ACCESS string.