Ask HN: How to display Different version of Same Website for Different Users

I was wondering if it's possible to create a website which acts like this :

Person X goes to example.com and sees one version Person Y goes to example.com and sees another version of the site ( which might differ thematically or functionally as well ).

How easy or hard is it to achieve this? I know some websites use ISP information to redirect customers away (eg: Pandora) - but it's only a static webpage. Is it possible to have two fully functional sites running simultaneously for different sets of users?