It would be very difficult to accept for Wade, I think, if Lebron signed in Miami. He's used to the Heat being "his" team. Having another star come in and trump him would be hard to take. With the Shaq situation, Wade was younger in his career; now, he's an established superstar.
Lebron, too, might have a concern that Wade might suck up some of his glory.
Plus, they're both the types of guy who play best with the ball in their hands. Certainly, a team with both of them would be mega-talented, but I also think they'd have some significant growing pains.