When you use register_taxonomy(), the second argument is the object_type argument, which accepts either a string or an array. You can pass this an array of objects to apply the taxonomy to, and it will be common between those. So, for example: array( 'cpt_slug', 'post' ) would give you a common taxonomy between cpt_slug and post.
If you want to add the existing taxonomies to a new post type, you can use register_taxonomy_for_object_type()