WPF: Can i set the width of an element by percentages?

Say I have 2 buttons in an Element and I want to set the 2 elements to always fill up 1/2 width of its containing element each, can i do that?

UPDATE

why cant i do something like

<StackPanel Orientation="Horizontal" Grid.Row="0">
    <Button Content="Click me" Command="{Binding ClickCommand}" Width="1*" />
    <Button Content="Exit" Command="{Binding CloseCommand}" Width="1*" />
</StackPanel>

why doesnt the 1* work in this context? i get the error

Cannot convert "1*"