Hi,
There is a flag on the mp_Sites table IsServerAdminSite, if set to true then the site can manage other sites, if set to false it cannot. By default only the first site created has this flag as true, but you could manually change it on another site directly in the db. However the default site will always be the first site, that is if a host name or folder name is used that is not mapped to a specific site, then it will resolve to the default site which is going to be the first site regardless of whether you make another site a server admin site.
Hope it helps,
Joe