Two different sites, two installations, no database or core sharing
put your drupal sites in separate subdirectories of htdocs, e.g .../htdocs/site1, ...htdocs/site2
access your sites using the mamp localhost url, e.g. http://localhost/site1


If you know how, you can also create virtualhosts in /Applications/MAMP/conf/apache/httpd.conf and edit /etc/hosts on your mac, so that you can access the sites with whichever url you choose, such as:

http://site1.local


Databases: just upload your databases using MAMP's phpmyadmin, and edit your sites' settings.php to use the respective databases.