With a domain aliasing you can have two domain names such as "a.com" and "b.com" pointing to the same physical web site and email services. Both web sites point to the same root folder and use the same default page. The single FTP or FrontPage user assigned to the account is used to manage all site content.
It is possible to have different web pages for different aliases using a ASP code snippet on the main default page (at the root of the site). To differentiate between the domain name used to access the web site you need to add a bit of ASP code in your default page to detect the domain name used and then redirect the user request to the appropriate sub-folders. Example code for doing this can be found in our support FAQ.
Regarding email, a.com and b.com are exactly the same email domains. Mail sent to either email@example.com or firstname.lastname@example.org is delivered to the same physical INBOX on the mail server. To retrieve the mail, you can access this single INBOX through the "me" user account using either domain name.
Since all aliased email accounts resolve to the single underlying list of email users it is not possible to have separate user accounts unique to each alias. To do this, instead of an alias, you need to set up a physically separate domain.