The Two Way Picker allows you to manage many to many, and one to many relationships across multiple nodes. This can reduce dependencies on tree structure and makes managing relationships on either side easy. A good example where the Two Way Picker excels is in a product category, product relationship. You may choose which product category a product belongs to from either the product category or product node.
Since the two way picker was built off of Umbraco's Multinode tree picker, members, media and content are all supported.