TwinColorBorder.xaml

<Page
    x:Class="XamlTraining.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:XamlTraining"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Grid Background="{ThemeResource FocusVisualWhiteStrokeThemeBrush}">
        <Border Grid.Column="0" Width="400" Height="200" CornerRadius="50" BorderThickness="5" BorderBrush="Red" Background="Red">
            <Grid>
                <Grid>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition />
                        <ColumnDefinition />
                    </Grid.ColumnDefinitions>
                    <Border Margin="-5"  Width="400" Height="200" CornerRadius="50" BorderThickness="5" BorderBrush="Blue" Background="Blue"/>
                </Grid>
                <TextBlock Text="サンプル" HorizontalAlignment="Center" VerticalAlignment="Center"/>
            </Grid>
        </Border>
    </Grid>
</Page>