Wpf: how to equally distribute the available height among 2 expander controls in grid without any gray area

Question

I have a Floating SplitPane, in which there are 3 expanders. The first expander has max of 3 rows of controls (label and textboxes) to display and needs to be displayed without a scroll.

And second and third expanders have one DataGrid each and the number of rows depends on filters available in the Pane. For example: there is Favorites button at the Pane level, clicking on this button will show only favorite marked rows in the dataGrid and thus, it may have less number of rows.

<UserControl>  
    <Grid x:Name="Root" >  
        <Grid.RowDefinitions>  
            <RowDefinition Height="Auto" />  
            <RowDefinition Height="*" />  
        </Grid.RowDefinitions>  
        <!--  Variables Expander  -->  
        <Expander x:Name="GeneralExpander" >  
                <!-- Two labels and Textboxes -->  
        </Expander>  
        <Grid x:Name="ExpanderGrid" Grid.Row="1" VerticalAlignment="Top">  
            <Grid.RowDefinitions>  
                <RowDefinition Height="Auto" />  
                <RowDefinition Height="*" />  
            </Grid.RowDefinitions>  
            <Expander x:Name="ParamsExpander" Collapsed="OnParamsExpanderCollapsed" Expanded="OnParamsExpanderExpanded" IsExpanded="False" >  
                <view:ParamsView x:Name="ParamsView" />  
            </Expander>  
            <Expander x:Name="VariablesExpander" Grid.Row="1" Collapsed="OnVarsExpanderCollapsed" Expanded="OnVarsExpanderExpanded" IsExpanded="true">  
                <view:VarsView x:Name="VarsView" />  
            </Expander>  
  
        </Grid>  
    </Grid>  
</UserControl>  

My requirement is to, equally distribute the available space between 2nd and 3rd expanders (when in expanded state). For which I have below events in code-behind.

See the screenshot to see how it is behaving currently:

QUESTION: But when I click on "Favorites" Star button, if any of them need only less height to display datagrid; the other expander needs to occupy the empty space instead of showing grey area.

Please give me some idea on how to achieve this (either from XAML or code-behind) and let me know if any other details required! Thanks in Advance.

What I have tried:

I have tried Collapsed and Expanded events to set "50*" for both the rows to equally distribute height. But it fails when one of the expander have very less records to show. It shows Gray area, as "50*" looks like not a right setting.

private void OnParamsExpanderExpanded(object sender, RoutedEventArgs e)  
{  
    if (this.paramLoadRequired)  
    {  
        this.paramLoadRequired = false;  
        // Populate respective datagrid rows  
    }  
  
    if (VarExpander.IsExpanded)  
    {  
        // Expander Grid rows (Params and Vars expanders) with equal height  
        ExpanderGrid.RowDefinitions[0].Height = new GridLength(50, GridUnitType.Star);  
        ExpanderGrid.RowDefinitions[1].Height = new GridLength(50, GridUnitType.Star);  
        return;  
    }  
  
    ExpanderGrid.RowDefinitions[0].Height = GetStarGridLength();  
    ExpanderGrid.RowDefinitions[1].Height = GetAutoGridLength();  
}  
  
private void OnVarsExpanderExpanded(object sender, RoutedEventArgs e)  
{  
    if (ParamsExpander.IsExpanded)  
    {  
        // Expander Grid rows (Params and Vars expanders) with equal height  
        ExpanderGrid.RowDefinitions[0].Height = new GridLength(50, GridUnitType.Star);  
        ExpanderGrid.RowDefinitions[1].Height = new GridLength(50, GridUnitType.Star);  
        return;  
    }  
  
    ExpanderGrid.RowDefinitions[1].Height = GetStarGridLength();  
    ExpanderGrid.RowDefinitions[0].Height = GetAutoGridLength();  
}  
  
private void OnVarsExpanderCollapsed(object sender, RoutedEventArgs e)  
{  
    // When Vars collapsed, need to adjust the height of Params expander to occupy full available height  
}  
  
private void OnParamsExpanderCollapsed(object sender, RoutedEventArgs e)  
{  
    // When Params collapsed, need to adjust the height of Vars expander to occupy full available height  
}