You should simply have separate folders on the host site under your main account. I have several sites under one package and when I log in (via WISE-FTP) I simply open the folder of the domain I'm working on.

In your hosting account set up you point the domain name to the appropriate folder.

To the outside world they appear as separate and distinct sites.
e.g., maggievalleyweddings.com, maggievalleycottage.com

+
|--maggievalleyweddings
| |
| images
| misc
|
--maggievalleycottage
| |
| images
| misc