Please refer to the code below.
$('#dg').datagrid({
onBeginEdit: function(index,row){
var ed1 = $(this).datagrid('getEditor', {index:index,field:'col1'});
var ed2 = $(this).datagrid('getEditor', {index:index,field:'col2'});
$(ed1.target).combobox({
onChange: function(value){
$(ed2.target).combobox('reload', {
value: value
})
}
})
}
})