Bind to all of the SelectedItems in a ListBox set to multiple select mode.

ListBoxExtensions.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;

namespace WPFStuff
{
    public static class ListBoxExtension
    {
        public static IList GetSelectedItems(DependencyObject obj)
        {
            return (IList)obj.GetValue(SelectedItemsProperty);
        }

        public static void SetSelectedItems(DependencyObject obj, IList value)
        {
            obj.SetValue(SelectedItemsProperty, value);
        }

        // Using a DependencyProperty as the backing store for SelectedItems.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty SelectedItemsProperty =
            DependencyProperty.RegisterAttached("SelectedItems", typeof(IList), typeof(ListBoxExtension), new PropertyMetadata(Setup));

        private static void Setup(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            // Prevent duplicate event handlers
            if (e.OldValue != null)
                ((Selector)d).SelectionChanged -= SelectionChanged;

            var items = e.NewValue as IList ?? new object[0];

            // To sync up initial values
            foreach (var item in items)
                (d as ListBox).SelectedItems.Add(item);

            ((Selector)d).SelectionChanged += SelectionChanged;
        }

        static void SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            var items = GetSelectedItems(sender as DependencyObject);
            if (items == null)
                return;

            foreach (var item in e.RemovedItems)
                items.Remove(item);

            foreach (var item in e.AddedItems)
                items.Add(item);
        }
    }
}