using System;
using System.Windows;
using System.Windows.Controls;

namespace Test
{
    public class ComplexControl: UserControl
    {
            static ComplexControl()
            {
               DefaultStyleKeyProperty.OverrideMetadata(typeof(ComplexControl), new FrameworkPropertyMetadata(typeof(ComplexControl)));
            }

            public object Left
            {
                get { return Content; }
                set { Content = value; }
            }

            public object Right
            {
                get { return (object)GetValue(RightProperty); }
                set { SetValue(RightProperty, value); }
            }

            public static readonly DependencyProperty LeftProperty =
                DependencyProperty.Register("Left", typeof(object), typeof(ComplexControl), new UIPropertyMetadata(null));
            public static readonly DependencyProperty RightProperty =
                DependencyProperty.Register("Right", typeof(object), typeof(ComplexControl), new UIPropertyMetadata(null));
    }
}

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:local="clr-namespace:Test">
    <ControlTemplate x:Key="Decorator" TargetType="local:ComplexControl">
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition/>
                <ColumnDefinition/>                
            </Grid.ColumnDefinitions>
            <ContentPresenter Grid.Column="0" ContentSource="Left"/>
            <ContentPresenter Grid.Column="1" ContentSource="Right"/>
        </Grid>
    </ControlTemplate>
</ResourceDictionary>

<Window x:Class="Test.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:Test" 
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Themes/Generic.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Window.Resources>
    <local:ComplexControl Template="{StaticResource Decorator}">
        <local:ComplexControl.Left>
            <Button Content="Left"/>
        </local:ComplexControl.Left>
        <local:ComplexControl.Right>
            <Button Content="Right"/>
        </local:ComplexControl.Right>
    </local:ComplexControl>
</Window>