Edit in JSFiddle

var data = [];
for (var i = 0; i < 20; i++) {
    data.push({
        data1: parseInt(i / 10),
        data2: i
    });
}

var grid = new wijmo.grid.FlexGrid('#FlexGrid');
grid.itemsSource = data;

grid.allowMerging = wijmo.grid.AllowMerging.All;
grid.columns.getColumn("data1").allowMerging = true;

<div id="FlexGrid" style="height: 350px;"></div>