In this case we can suggest you to use autosize columns to fit grid's width FitToWidth mode of Columns.
Just specify Column.FitMode = ColumnFitMode.Spring for some column in the left and right grids and add to Columns.Options ColumnsOptions.FitToWidth option:
[c#]
leftGrid.Columns.Items[0].FitMode = ColumnFitMode.Spring;
rightGrid.Columns.Items[0].FitMode = ColumnFitMode.Spring;
leftGrid.Columns.Options |= ColumnsOptions.FitToWidth;
rightGrid.Columns.Options |= ColumnsOptions.FitToWidth;