When a row begins to be edited, the 'onBeginEdit' event fires. You can get the current editors and create a dependency between these editors.
$('#dg').datagrid({
onBeginEdit:function(index,row){
var c1 = $(this).datagrid('getEditor',{index:index,field:'GLL_GER1'});
var c2 = $(this).datagrid('getEditor',{index:index,field:'GLL_GER2'});
var v1 = $(c1.target).combobox('getValue');
$(c1.target).combobox({
url: ...,
value: v1,
onChange: function(value){
var url = 'ueln.act.php?frm=BRE&act=L&lkp=ZUG&key=' + value;
$(c2.target).combobox('reload',url);
}
});
}
})