With all the linux shared hosting plans we are offering one domain name hosting (e.g. main domain name in your case) only. The entire resource allocated to your main domain name cannot be shared with other hosting domains.
To add second domain name to the same package, you can either purchase a new hosting plan (linux personal web hosting) or you can purchase a domain name from our registrar along with the domain forwarding service.
In the 2nd option, our registrar host your website on thier server and allow you to specify where to redirect all the queries of this domain name. For example, you can specify redirect attribute as http ://site2.mainddomainname.com/ or something like that.