config.yml

assetic:
  write_to: %kernel.root_dir%/../web/
  filters:
    compass:
      apply_to: "/\.s[ac]ss$/"

index.html.twig

{% stylesheets
  "@SNWebBundle/Resources/assets/css/main.scss"
  "@OtherBundle/Resources/assets/css/main.sass"
  "@GeneralBundle/Resources/assets/css/main.css"
  output='css/output.css' 
%}
    <link rel="stylesheet" type="text/css" href="{{ asset_url }}" />
{% endstylesheets %}