OK, the solution wasn't any of my previous theories .....
was that the data i was incoming in the second Datastore was in the incorrect format...
i inserted a Field.data in LoadData() method.
well the LoadData() method was waiting to recieve a simple Multidimensional Array (oversight it ... too much cafeine
)
by the way i'm changed my second datastore for a SimpleStore to symplify my declaration
This is my final code
Code:
{
title : '...',
xtype : 'grid',
id : 'grdXXXX',
store : new Ext.data.SimpleStore({
data : [['Dummy Prop','Dummy Val']],
fields : [{name: 'tname', mapping: 0, type: 'string'},
{name: 'tvalue', mapping: 1, type: 'string'}]
}),
columns : [
{header : "Property", width: 200, sortable: true, dataIndex: 'tname'},
{header : "Value", width: 120, sortable: false, dataIndex: 'tvalue'}
],
viewConfig : {
forceFit : true,
emptyText : '...'
}
}]
}
and here comes the magic 
Code:
store : new Ext.data.Store({
proxy : new Ext.data.HttpProxy({url : 'detObject.asp'}),
id : 'tname',
reader : new Ext.data.XmlReader({
record : 'data'
},
[{name: 'name', mapping: '@tname'},
{name: 'value', mapping: '@tvalue'}]
),
listeners :{
'load' : function( oDts, oRecs, oOpts){
var cont,cont2;
var oFld = new Array();
cont2=0;
Ext.getCmp('grdXXXX').store.removeAll(); //legacy code ... maybe rendundant
for(cont=0;cont < oRecs.length;cont++ ){
if(oRecs[cont].get('nombre') == 'XXXX'){
Ext.getCmp('tpGral').form.findField("txtXXXX").setValue(oRecs[cont].get('tvalue'));
continue;
}
else{
oFld[cont2]= [oRecs[cont].get('tname'),oRecs[cont].get('tvalue')];
cont2++;
}
}
Ext.getCmp('grdXXXX').store.loadData(oFld,false);
oFld=null;
}
}
}),
in short Words it's offcially SOLVED