Man Utd could afford both of them, theyr'e the richest club in teh world remember. But like laready mentioned, Fergusson will probably find it too expensive. He's already got a winning team, why an extra 2 players with huge price-tags???
On the other hand, neither player has established themselves in a very good league yet, so they can't be worth that much. Man Utd will come with a few offers, until something around the 6-7 million pound mark, and Feyenoord can't resist, they'll let Dudek go. A number of other players want to leave Feyenoord, eventually Feyenoord has all got to let them go. Big clubs are interested in them, Feyenoord is playing poorly, you can't kee the players there against their will, the players lack of respect has already psychologically blown one coach, there could be a next. Feyenoord need to sell the players that have been targeted by big clubs, and then try to buy a handful of talents from the Dutch league for next season. And 20 million for Van Nistelrooij is preposterous, he's worth 15 mil pounds at the most.