Vista - A question about ItemsControl

I wrote a class "MyContorl" based on ItemsControl.
In the controltemplate, I use a Grid(1*2) to place the items.
Then I put two buttons and a gridsplitter into MyContorl.
I want to use the gridsplitter to change the size of the tow button.
But it can't work.
I want it work like the last two button and gridsplitter

/*-----------MyContorl Class--------*/
namespace ItemsControlTest
{
class MyControl : ItemsControl
{
public MyControl()
{
}
}
}



<Window x:Class="ItemsControlTest.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="ItemsControlTest" Height="300" Width="300"
xmlns:local="clr-namespace:ItemsControlTest"
>
<Window.Resources>
<!--/////////////////////////////////Contorl
Template/////////////////////////////-->
<Style x:Key="{x:Type local:MyControl}" TargetType="{x:Type
local:MyControl}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type
local:MyControl}">
<Grid x:Name="MyGrid" IsItemsHost="True">
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Window.Resources>

<StackPanel>
<local:MyControl Width="200" Height="200">
<Button Grid.Row="0" Height="Auto" Width="Auto"/>
<Button Grid.Row="1" Height="Auto" Width="Auto"
Margin="0,5,0,0"/>
<GridSplitter Grid.Row="1" HorizontalAlignment="Stretch"
Height="5" VerticalAlignment="Top"/>
</local:MyControl>
<!--///////////////////the result i
want//////////////////////////////-->
<Grid Width="200" Height="200" Margin="0,10,0,0">
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Button Grid.Row="0" Height="Auto" Width="Auto"/>
<Button Grid.Row="1" Height="Auto" Width="Auto"
Margin="0,5,0,0"/>
<GridSplitter Grid.Row="1" HorizontalAlignment="Stretch"
Height="5" VerticalAlignment="Top"/>
</Grid>
</StackPanel>
</Window>