Could you please try to separate the PaneGroup in different SplitContainer, one for each group. Then, manipulate the MaxHeight of the SplitContainers instead of the PaneGroups. Or you could set the following attached property: telerik:ProportionalStackPanel.RelativeSize
Initially, the Docking control supports relative sizing, so you have to keep in mind that the SplitContainers will try to honor this.
Hope this information helps.
the Telerik team
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