You should bind detail section dynamically according to what key is selected from ComboBox.
Your path will be dynamic. Assuming principalset key is static 0, your binding path will be something like that in your Change event of ComboBox.
var oItem = oEvent.getParameter("item") || oEvent.getSource();
var sKey= oItem.mProperties.selectedKey;
...
...
objDetalhes.bindItems("/PrincipalSet(0)/Relatorios(" + sKey + ")/DetalhesSet" , oTemplate);