I missed the multi-row part of your question. Try handing the grid.Model.SlectionChanged event, and there select the other row while a single row has been selected.
private void Model_SelectionChanged(object sender, GridSelectionChangedEventArgs e)
{
if(e.Range.IsRows && e.Range.Height == 1)
{
int row = - 1;
if(e.Range.Top % 2 == 0)
{
row = e.Range.Top + 1;
}
else
{
row = e.Range.Top - 1;
}
this.gridDataBoundGrid1.Selections.SelectRange(GridRangeInfo.Cells(row, 1, row, this.gridDataBoundGrid1.Model.ColCount), true);
}
}