typedef fst::StdVectorFst GRAPH;
GRAPH _graph1,  _graph2;
//...
fst::StdOLabelLookAheadFst graph1Look(_graph1);
std::auto_ptr<GRAPH> graph2TrueRelabel(_graph2.Copy(true));
fst::LabelLookAheadRelabeler<GRAPH::Arc>::Relabel(graph2TrueRelabel.get(), graph1Look, true);
fst::ArcSort(graph2TrueRelabel.get(), fst::StdILabelCompare());
std::auto_ptr<GRAPH> res(new GRAPH);
fst::Compose(graph1Look, *graph2TrueRelabel.get(), res.get());