SplitContainer and children width

Is there any way to set a width for a child of a SplitContainer in a docksite containing ToolWindowContainers? In my testing, it seems splits always divide the children evenly, even if one child is given a specific width. In XAML, I want to give one child (another SplitContainer) a particular width, and have the other(s) fill in the remaining space.

I have a somewhat standard example of 3 ToolWindows split so one takes up half horizontally, and the other two take the remaining half, split vertically. Below I've pasted the example setup for my dock site.

Thanks!

<docking:DockSite>
	<docking:SplitContainer Orientation="Horizontal">
		<docking:ToolWindowContainer>
			<docking:ToolWindow Title="Left">
				<ScrollViewer>
					<TextBlock TextWrapping="Wrap">... example text</TextBlock>
				</ScrollViewer>
			</docking:ToolWindow>
		</docking:ToolWindowContainer>
		<!-- 
			I want this split container to have a width, or take up a smaller percentage, not 50%.
			If working with a Grid, this would be done as part of the column definition.
		  -->
		<docking:SplitContainer Orientation="Vertical">
			<docking:ToolWindowContainer>
				<docking:ToolWindow Title="Upper Right">
					<ScrollViewer>
						<TextBlock TextWrapping="Wrap">... example text</TextBlock>
					</ScrollViewer>
				</docking:ToolWindow>
			</docking:ToolWindowContainer>
			<docking:ToolWindowContainer>
				<docking:ToolWindow Title="Lower Right">
					<ScrollViewer>
						<TextBlock TextWrapping="Wrap">... example text</TextBlock>
					</ScrollViewer>
				</docking:ToolWindow>
			</docking:ToolWindowContainer>
		</docking:SplitContainer>
	</docking:SplitContainer>
</docking:DockSite>