Это можно сделать, вставив сетку или панель стека в элемент управления границей, как показано ниже:
<Border CornerRadius="5,5,0,5" BorderThickness="2" BorderBrush="Black" HorizontalAlignment="Center" Width="100" Height="100" VerticalAlignment="Center">
<StackPanel>
<StackPanel.Background>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="Black" Offset="0"/>
<GradientStop Color="#FF030FC6" Offset="0.2"/>
</LinearGradientBrush>
</StackPanel.Background>
</StackPanel>
</Border>
<Border CornerRadius="5,5,0,5" BorderThickness="2" BorderBrush="Black" HorizontalAlignment="Left" Width="100" Height="100" VerticalAlignment="Center" Margin="68.833,0,0,0">
<Border.Background>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="Black" Offset="0"/>
<GradientStop Color="#FFE90D0D" Offset="1"/>
</LinearGradientBrush>
</Border.Background>
<Grid/>
</Border>