Major ODATA-24 simplifies relationships by moving association information into the navigation property. The missing piece of information; the entityset in which to find the related entity, is added through a "navigationpropertybinding" element within the EntitySet definition.
There are cases where the entity set of the target entit(ies) may not be known (i.e., containment, or when the targets may come from multiple different entitysets). To address these cases, the navigationpropertybinding was made optional in ODATA-24, to be provided when the target entities came from the same entityset.
The application of ODATA-24 contemplates an intermediate state, where the target entities may come from a fixed set of multiple entity sets, by allowing multiple navigationpropertybindings for the same navigation property. However, this information doesn't seem very useful, as the client wouldn't know, for any particular instance, which of the multiple entitysets the instance belonged to. Thus, allowing multiple navigationpropertybindings for the same navigation property seems to add unnecessary complexity for little or no added benefit.