.htaccess URL rewrite without changing the domain name

I want to transfer users from one to the othe开发者_如何学Pythonr without knowing them.

like www.domain1.com/folder to www.domain2.com/folder, but URL remain same www.doamin1.com/folder.

I'm sure it's possible with .htaccess but don't know how to do that.

Reason to do this just want to secure my application, don't want to run from client's hosting.

Really appreciate your help.