Code first EF 6 One to many to one

I am trying to use GraphDiff to update a hierarchical entity graph.

I have my Availability which has a number of Items, and also a booking which eventually wants to select some of these.

public class Availability{
//...
public virtual List<Item> Items { get; set; }
//...
}

public class Booking{
//..
public virtual List<Item> Items {get;set;}
//..
}

public class Item{
//..
public int ID {get;set;}
//..
}

public class MyContext : DbContext{
    protected override void OnModelCreating(DbModelBuilder modelBuilder){
        modelBuilder.Entity<Booking>().HasMany(b => b.Items).WithOptional(e => e.Booking).WillCascadeOnDelete(false);
        modelBuilder.Entity<Availability>().HasMany(ar => ar.Items).WithOptional(e => e.Availability).WillCascadeOnDelete(false);
    }
}

what I am trying to achieve is, rather than - when I assign an Item to a booking form the availability - EF generating 2 records, it generates a single record with 2 foreign keys in it

the DB table does have 2 foreign key columns in it - and I am updating like this

public ActionResult SaveItems(Availability model){
    Booking b = db.Bookings.Include(bk=>bk.Items).Include(...).singleOrDefault(bk=>bk.ID == model.ID);

    b.Items = model.Items;

    db.UpdateGraph(b, map => map
                        .OwnedCollection(bk=>bk.Items, with => with
                            .OwnCollection(i=>i.ChildList)));
    db.SaveChanges();
}

I am assuming that there is something in the fluent mappings that may be able to make this work, but I cant seem to work it out

I have also tried adding this to the OnModelCreating to no avail.

modelBuilder.Entity<Item>().HasOptional(e => e.Booking).WithMany(b => b.Items).WillCascadeOnDelete(false);
modelBuilder.Entity<Item>().HasOptional(e => e.Availability).WithMany(a => a.Items).WillCascadeOnDelete(false);

is this even possible ?