Convert two existing sites to multisite
Yes. Configure an apache virtual host for each site. They look something like this:

&lt;VirtualHost *:80&gt;
    DocumentRoot "/path/to/drupal/install"
    ServerName mysite.example.com
&lt;/VirtualHost&gt;

&lt;VirtualHost *:80&gt;
    DocumentRoot "/path/to/drupal/install"
    ServerName myothersite.example.com
&lt;/VirtualHost&gt;


Every virtual host in the multisite must be configured with the same DocumentRoot. The ServerName directive should match what you have used as the respective site's key in sites.php. Note that in this case *:80 signifies this virtual host listens on every IP of this server.