Possible to share assets, templates and plugins across two domains but not all content?
As long as your sites are hosted on the same server, you can easily share templates and plugins between them by defining the appropriate path constants in the index.php files for your sites:

define('CRAFT_PLUGINS_PATH', '/absolute/path/to/your/plugins/folder');
define('CRAFT_TEMPLATES_PATH', '/absolute/path/to/your/templates/folder');


As for your resources (i.e. CSS and JavaScript files), symbolic linking is an option. However, I would recommend following Tom Bauer's suggestion in his answer – pick one domain and serve all assets from that domain's public folder, or even better: host the resources on a CDN.