Code

void ArrangeTree(int levelToSplit, DiagramNode root)
{
	List<DiagramLink> tempLinks = new List<DiagramLink>();
	List<DiagramNode> levelNodes = new List<DiagramNode>();
	GetLevel(levelToSplit, levelNodes, 0, root);

	for (int i = levelNodes.Count / 2; i < levelNodes.Count; ++i)
	{
		DiagramNode node = levelNodes[i];
		tempLinks.Add(diagram.Factory.CreateDiagramLink(
			levelNodes[i - levelNodes.Count / 2], node));
		node.IncomingLinks[0].IgnoreLayout = true;
	}

	// run it with your current settings
	new TreeLayout().Arrange(diagram);

	foreach (DiagramLink link in tempLinks)
	{
		link.Destination.IncomingLinks[0].IgnoreLayout = false;
		diagram.Links.Remove(link);
	}
}

void GetLevel(int level, List<DiagramNode> leveNodes, int currentLevel, DiagramNode node)
{
	if (currentLevel == level)
	{
		leveNodes.Add(node);
	}
	else
	{
		foreach (DiagramNode child in node.Query("outlinks/destination"))
			GetLevel(level, leveNodes, currentLevel + 1, child);
	}
}