Pantheon only supports one application codebase and one database per site. We do not support database prefixes or Drupal multisite. Our workflow, backup, and deployment tools only perform as expected given the standard Pantheon setup.
While we can't give specific recommendations for your sites, we can give general best practices for having two sites under one domain. This is a known CMS problem, as it is a very common issue for which many strategies and workarounds exist. To read more about the issue, see Multi-headed Drupal.
- Separate installs with redirection to a subdomain
- Features-based installs with Custom Upstreams and redirection to a subdomain
- Domain access on a single site install. For a Drupal site, try the Subfolders Domain (sub domains) module but there is no guarantee it will work.
- Organic Groups