myapp.conf

<VirtualHost *>
  ServerName mydomain.local
  DocumentRoot /home/matthew/myapp/public
  RailsEnv development
</VirtualHost>

<VirtualHost *>
  ServerName somethingelse.local
  DocumentRoot /home/matthew/myapp/public/other_domain
  #PassengerAppRoot /home/matthew/myapp
  #RailsEnv development

  ProxyPass / http://mydomain.local/other_domain/
  ProxyPassReverse / http://mydomain.local/myapp/

  #RewriteEngine on
  #RewriteRule ^(.*)$ http://mydomain.local/myapp$1 [P]

</VirtualHost>