Managing Multiple selection in View Model (.NET Metro Style Apps)

<Page
    x:Class="MultiselectSample.MainPage"
    IsTabStop="false"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:MultiselectSample"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:i ="using:WinRtBehaviors"
    xmlns:custom="using:WinRtExt.Behavior"
    mc:Ignorable="d">

    <Page.Resources>
        <DataTemplate x:Key="textBlockDataTemplate">
            <TextBlock Width="100" Margin="10" Text="{Binding}"></TextBlock>
   
        </DataTemplate>
    </Page.Resources>
    <Grid Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
        <StackPanel Orientation="Horizontal" HorizontalAlignment="Center" VerticalAlignment="Center">
            <GridView SelectionMode="Multiple" ItemsSource="{Binding Items}" BorderBrush="White" BorderThickness="2" ItemTemplate="{StaticResource textBlockDataTemplate}">
            <i:Interaction.Behaviors>
                <custom:MultiSelectBehavior SelectedItems="{Binding SelectedItems, Mode=TwoWay}">
                    
                </custom:MultiSelectBehavior>
            </i:Interaction.Behaviors>
            </GridView>
            <Rectangle Width="20"></Rectangle>
            <ListView SelectionMode="Multiple" ItemsSource="{Binding Items}"  BorderBrush="White" BorderThickness="2"  ItemTemplate="{StaticResource textBlockDataTemplate}">
                <i:Interaction.Behaviors>
                    <custom:MultiSelectBehavior SelectedItems="{Binding SelectedItems, Mode=TwoWay}">

                    </custom:MultiSelectBehavior>
                </i:Interaction.Behaviors>
            </ListView>
        </StackPanel>
    </Grid>
</Page>