That is actually very easy:
<grid>
<grid.columndefinitions>
<columndefinition width="100" />
<columndefinition width="100" />
<columndefinition width="100" />
</grid.columndefinitions>
<grid.rowdefinitions>
<rowdefinition height="25" />
<rowdefinition height="25" />
<rowdefinition height="25" />
<rowdefinition height="25" />
<rowdefinition height="25" />
</grid.rowdefinitions>
<border grid.column="0">
Grid.Row="0"
Grid.RowSpan="2"
BorderThickness="1"
BorderBrush="Black"
Padding="3">
<textblock text="Row1" />
</border>
<border grid.column="0">
Grid.Row="2"
Grid.RowSpan="2"
BorderThickness="1"
BorderBrush="Black"
Padding="3">
<textblock text="Row2" />
</border>
<border grid.column="1">
Grid.Row="0"
BorderThickness="1"
BorderBrush="Black"
Padding="3">
<textblock text="SubRow1" />
</border>
<border grid.column="1">
Grid.Row="1"
BorderThickness="1"
BorderBrush="Black"
Padding="3">
<textblock text="SubRow2" />
</border>
</grid>