As said previously, check your HTACCESS on both sites, and your apache virtual host. In theory, you can imbricate websites... But it's not recommended and easily maintainable.

But the simpler and far more recommended will be to reorganise your sites : If you want to keep 2 Drupal separately :

• /var/www/html/SiteNAME1
• /var/www/html/SiteNAME2

If you can have a multi site, you can check https://www.drupal.org/docs/8/multisite

As said previously, check your HTACCESS on both sites, and your apache virtual host. In theory, you can imbricate websites... But it's not recommended and not easily maintainable.

But the simpler and far more recommended way will be to reorganise your sites/folders : If you want to keep 2 Drupal separately :

• /var/www/html/SiteNAME1
• /var/www/html/SiteNAME2

If you are interested in multi site architecture, please check https://www.drupal.org/docs/8/multisite