Can Frank Schleck and Denis Menchov win the vuelta?
Technically, NO, Frank Schleck AND Denis Menchov cannot both win the Vuelta. There can be only one winner.
On the practical side, I don't think that Menchov has a chance. In the cleaning of the Peloton since his Giro win, his performances have fallen farther and farther behind. And, like we are seeing from Sastre too, performing at a top level in 2 GT's is very difficult (ask Wiggo), but trying to complete 3 and winning the last one is nearly impossible.
As for Schleck, I think that he has the better chance, especially if he prepared anything like Andy for the Tour. Frank is very fresh with minimal race days in him so far this year after crashing out on the 4th stage of the Tour. In the world of riding into shape during the race, Frank's body will likely respond much better than a Menchov of Sastre because it is not so overworked for the year.
Of course, my thoughts could be far from what really happens. We'll see.