Bush is the perfect thunder to go with the lightning of McFadden because they balance each other well. McFadden isn't as powerful as Bush but he has power so both of these men can wear the defense down.
Bush doesn't have the mercurial speed of McFadden but he can go the distance to complete a big run. Both men catch the ball well and protect the quarterback so the defense can't tell what's coming with either guy in there.
This combination shouldn't be broken up because you don't give up too much of one thing to get the other when you substitute these men. Plus, as I said before, you just don't give up good football players because they are commodities you can broker.
Instead, I prefer that Bush stay in Raider Nation to continue to build the "bully."