Newbie question about Multi-domains plugin

Hi, domain1.com is my main domain and domain2.com is my secondary domain.

Hi, can I know whether the username.domain2.com is using the same DB as domain1.com or are they totally different DB? Do I have to install a new WP in domain2.com?

If I have to install a new WP in domain2.com, what plugin do I have to set in the domain2.com WP?

Thanks