You can custom the 'loadFilter' function to construct a new combined field, and then set the 'textField' property to this field name. Please look at this code.
<select class="easyui-combogrid" style="width:100%" data-options="
panelWidth: 500,
idField: 'itemid',
textField: 'myfield',
url: 'datagrid_data1.json',
method: 'get',
columns: [[
{field:'itemid',title:'Item ID',width:80},
{field:'productname',title:'Product',width:120},
{field:'listprice',title:'List Price',width:80,align:'right'},
{field:'unitcost',title:'Unit Cost',width:80,align:'right'},
{field:'attr1',title:'Attribute',width:200},
{field:'status',title:'Status',width:60,align:'center'}
]],
fitColumns: true,
label: 'Select Item:',
labelPosition: 'top',
loadFilter: function(data){
if ($.isArray(data)){
data = {total:data.length,rows:data};
}
$.map(data.rows, function(row){
row.myfield = row.itemid+':'+row.productname;
});
return data;
}
">
</select>