How to split a Grid row into two columns?

Two ways you can do it:

• Use nested layouts. Put another Grid in the third row, and have two columns in that sub-grid.

  <Grid>
      <Grid.RowDefinitions> ... </Grid.RowDefinitions>
      <ThingInFirstRow Grid.Row="0" />
      <ThingInSecondRow Grid.Row="1" />
      <Grid Grid.Row="2">
          <Grid.ColumnDefinitions>
              <ColumnDefinition />
              <ColumnDefinition />
          </Grid.ColumnDefinitions>
          <ThingInLowerLeft Grid.Column="0" />
          <ThingInLowerRight Grid.Column="0" />
      </Grid>
  </Grid>
  

• Stick with one Grid, give it two columns, and make the things in the first two rows span across both columns using ColumnSpan.

  <Grid>
      <Grid.RowDefinitions> ... </Grid.RowDefinitions>
      <Grid.ColumnDefinitions>
          <ColumnDefinition />
          <ColumnDefinition />
      </Grid.ColumnDefinitions>
      <ThingInFirstRow Grid.Row="0" Grid.ColumnSpan="2" />
      <ThingInSecondRow Grid.Row="1" Grid.ColumnSpan="2" />
      <ThingInLowerLeft Grid.Row="2" Grid.Column="0" />
      <ThingInLowerRight Grid.Row="2" Grid.Column="1" />
  </Grid>