Apply your requirement in a Style
DataTrigger
for RowDefinition
<Grid Name="LayoutRoot">
<Grid.RowDefinitions>
<RowDefinition Name="GridRow1">
<RowDefinition.Style>
<Style TargetType="{x:Type RowDefinition}">
<Setter Property="Height"
Value="*" />
<Style.Triggers>
<DataTrigger Binding="{Binding ElementName=expanderOne,
Path=IsExpanded}"
Value="False">
<Setter Property="Height"
Value="Auto" />
</DataTrigger>
</Style.Triggers>
</Style>
</RowDefinition.Style>
</RowDefinition>
<RowDefinition Name="GridRow2"
Height="*" />
</Grid.RowDefinitions>
<Expander x:Name="expanderOne"
Grid.Row="0">
<TextBox TextWrapping="Wrap"
VerticalScrollBarVisibility="Auto" />
</Expander>
<Expander Grid.Row="1">
<TextBox TextWrapping="Wrap"
VerticalScrollBarVisibility="Auto" />
</Expander>
</Grid>