FYI, Types plugin will have complete many-to-many post relationship in version 2.3 (two major versions from now).

https://wordpress.org/plugins/types/

The upcoming release of Types 2.2 already includes the foundations for this (some API that we're creating internally).

Types 2.3 will also have the GUI, so that users can declare one-to-many and many-to-many relationships between different post types.

We took inspiration from posts-2-posts plugin for our implementation, but it's not the same. In Types, there will be both an API, to use in PHP code, and a complete GUI, to set up things from the WordPress admin.