"A Secondary Domain Alias (a.k.a. A Domain Name Pointer or Domain Alias) is another domain name you can use to access the services at your main domain. In other words, you can use one domain name to point to the services of another domain.
For example, if you had the domain name "primary-domain.com" and also had the domain "secondary-domain.com", you could set up a website on the primary domain, then use both
www.primary-domain.com and
www.secondary-domain.com to get to the same single website. "
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http://support.alentus.com/dns_alias...&sNode=3&Exp=Y
It would appear that an Alias Domain or Domain Alias does not provide the same function as an addon domain. I think your only option to get make it a new host (or redirect). Then again there might be a way to make your index do this: If from first.com send to /public_html if from second.com send to /public_html/secondary Altho I'm not sure how you do this.