Okay, I actually am struggling because my button heights (65) and stroke thickness (3) is different from your example. Did you make these Paths in a Blend designer? What is the best way to make them?
https://i.imgur.com/IuLQ7CW.png
<Grid UseLayoutRounding="False">
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition Width="20" />
</Grid.ColumnDefinitions>
<Border
Grid.Column="0"
Margin="0,0,0,0"
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="3,3,0,3">
<ContentPresenter Margin="10,5,0,5" VerticalAlignment="Center" />
</Border>
<Path
Grid.Column="1"
Data="M0,0L19,0 0,65z"
Fill="{TemplateBinding Background}" />
<Path
Grid.Column="1"
Data="M0,2L17,1 0,63"
Stroke="{TemplateBinding BorderBrush}"
StrokeThickness="3" />
</Grid>
<Grid Margin="-15,0,0,0" UseLayoutRounding="False">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="20" />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<Path
Grid.Column="0"
Data="M20,0L25,60 0,65z"
Fill="{TemplateBinding Background}" />
<Path
Grid.Column="0"
Data="M20,2L2,58 60,63"
Stroke="{TemplateBinding BorderBrush}"
StrokeThickness="3" />
<Border
Grid.Column="1"
Margin="-0.5,0,0,0"
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="0,3,3,3">
<ContentPresenter Margin="0,5,5,5" VerticalAlignment="Center" />
</Border>
</Grid>
