2 domains on same login?

I am not sure how to explain this but here goes.
I have a domaine name hosted, a main domain name, but on the same account I have a second domaine “sleeping”.
I know I need two folder where each domain name point to and I would also liked them masked so you only see the domain and not the sub domain/folder in the url address.

So how do I do this?

1)In the root directory, create a .htaccess file that controls which of subfolders one goes to.
2)In the same file,create a URL rewrite that mask the redirect.

Thank you and sorry for my English
Atm there is no WP up, it is suppose to go on the second domain.