I want to throw this out there as an answer to the question. I have stumbled across a plugin that will take WordPress’s permissions to the next level.
It is called Press Permit. http://presspermit.com/extensions/pp-collaborative-editing/
Taken from Press Permit FAQ’s:
How does Press Permit compare to Capability Manager, User Role Editor and other role editor plugins?
Press Permit’s functionality is different from and complementary to a
basic role editor / user management plugin. In terms of permissions,
those plugins’ primary function is to alter WordPress’ definition of
the capabilities included in each role. In other words, they expose
lots of knobs for the permissions control which WordPress innately
supports. That’s a valuable task, and in many cases will be all the
role customization you need. Since WP role definitions are stored in
the main WordPress database, they remain even said plugin is
deactivated.Press Permit can assist you in turning the site-wide capability knobs
for desired post types. But it also supercharges your permissions
engine. Press Permit it is particularly useful when you want to
customize access to a specific post, category or term. Extension
plugins add collaborative editing control, file filtering and other
features which are not otherwise possible. The plugin will work with
your WP roles as a starting point, whether customized by a role editor
or not. Users of the PP Collaborative Editing extension can (after
activating advanced settings) navigate to Permissions > Settings >
Role Usage to see (or modify) how Press Permit is using your WP role
definitions. Press Permit’s modifications remain only while it stays
active.
It looks like Press Permit’s extension Collaborative Editing can accomplish restricting single posts to multiple authors or even groups.
This video explains more about the plugin.