I would like to redirect a new domain to a particular sub-folder under an old domain, but would like to have URL masking, so that the user will not see the old domain URL. To simplify what I am looking for, I'll give an example here...

Let the main domain account be: www.1.com
I have a subfolder in the main account, let it be: www.1.com/a/
I have a new domain name, let it be www.2.com, whose nameserver is set to the same server
I want www.2.com to point to www.1.com/a/ with URL masking (<- thats important) - ie., the user should not see www.1.com/a/ or any webpage under www.1.com in the address when accessing through www.2.com

What are the steps I need to follow to set it up through my cPanel/WHM ?
Or is it better to set it up by editing .htaccess file ?
One person suggested me to use frames, but isn't there any other way ?

Thanks in advance !