MainView.xaml

<Window x:Class="featurebased.MainView"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:featurebased"
        xmlns:FriendsList="clr-namespace:featurebased.FriendsList"
        xmlns:FriendDetail="clr-namespace:featurebased.FriendDetail"
        mc:Ignorable="d"
        Title="MainWindow"
        Height="350"
        Width="525"
        d:DataContext="{d:DesignInstance local:MainViewModel}">
    <Window.Resources>
        <ResourceDictionary Source="FriendDetail/FriendView.xaml" />
    </Window.Resources>
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="2*" />
        </Grid.ColumnDefinitions>
        <FriendsList:FriendsView Grid.Column="0"
                                 DataContext="{Binding FriendsViewModel}" />
        <ContentPresenter Grid.Column="1"
                          Content="{Binding FriendsViewModel.SelectedFriend}"
                          ContentTemplate="{StaticResource SelectedFriendDataTemplate}" />
    </Grid>

</Window>