Multi-site just separated content There is a lot of blog posts and tutorial about multi-site with content sharing.

But what about inverse of it? Multi-site with separated contents?

Exactly which tables should be shared?

here is my settings.php for sub-site:

$databases = array(
    'default' =&gt;
    array(
        'default' =&gt;
        array(
            'database' =&gt; 'xxx',
            'username' =&gt; 'xxx',
            'password' =&gt; 'xxx',
            'host' =&gt; 'localhost',
            'port' =&gt; '',
            'driver' =&gt; 'mysql',
            'prefix' =&gt; 'default_portal_',
            'prefix' =&gt; array(
                'default' =&gt; 'default_portal_',
                // Users
                'users' =&gt; '',
                'sessions' =&gt; '',
                'role' =&gt; '',
                'role_permission' =&gt; '',
                'authmap' =&gt; '',
                'blocked_ips' =&gt; '',

                // Blocks
                'block' =&gt; '',
                'block_custom' =&gt; '',
                'block_node_type' =&gt; '',
                'block_role' =&gt; '',

                // CKEditor
                'ckeditor_input_format' =&gt; '',
                'ckeditor_settings' =&gt; '',

                'comment_notify_user_settings' =&gt; '',

                // Configuration
                'date_formats' =&gt; '',
                'date_format_locale' =&gt; '',
                'date_format_type' =&gt; '',
                'field_config' =&gt; '',
                'field_config_instance' =&gt; '',
                'filter' =&gt; '',
                'filter_format' =&gt; '',
                'i18n_block_language' =&gt; '',
                'i18n_string' =&gt; '',
                'i18n_translation_set' =&gt; '',
                'image_effects' =&gt; '',
                'image_styles' =&gt; '',
                'languages' =&gt; '',
                'locales_source' =&gt; '',
                'locales_target' =&gt; '',
                'node_type' =&gt; '',
                'quicktabs' =&gt; '',
                'rdf_mapping' =&gt; '',

                // Support
                'support_assigned' =&gt; '',
                'support_client' =&gt; '',
                'support_priority' =&gt; '',
                'support_reference' =&gt; '',
                'support_states' =&gt; '',
                'support_ticket' =&gt; '',
                'support_ticket_comment' =&gt; '',

                // System
                'system' =&gt; '',
                'taxonomy_term_data' =&gt; '',
                'taxonomy_term_hierarchy' =&gt; '',
                'taxonomy_vocabulary' =&gt; '',
                'variable' =&gt; '',
                'variable_store' =&gt; '',

                // Views
                'views_display' =&gt; '',
                'views_view' =&gt; '',

                // WYSIWYG
                'wysiwyg' =&gt; '',
                'wysiwyg_user' =&gt; '',
            ),
        ),
    ),
);


EDIT

Above configuration works after cache clearing, but some tables should not shared: variable and variable_store because of site_title and site_slogan ...

EDIT

If I share block table, I should enable all themes in main site.

yet I have question to ensure which tables should share?
Yes , this is good idea, in one of my past work we do something like this, dedicated  drupal core but with separate content .