Different time, different system.
In 2012, Harden was coming off the bench and was the third option behind Durant and Westbrook in Scotty Brooks' iso heavy gameplan when they were on the floor together. Both players have earned MVPs on separate teams since then, and consider themselves to be the alpha.
Now they will be starting together in D'Antoni's pace-and-space three-palooza system and, knowing Pringles, he is gonna play them each 35-40 MPG...which means they have to co-exist even though they both are accustomed to having high usage. Who is going to demand the ball in crunch time of big games?
Not to mention that Westbrook is basically the opposite of Morey-ball. Terrible three-point shooter (but perfectly willing to chuck them), loves to shoot in mid-range, doesn't really move or cut without the ball, hangs around for rebounds instead of breaking out in transition, the list goes on and on. That would all be okay if he was just playing a role in spot minutes, but he is sure to have the 2nd-highest usage on the team.
Houston will still win lots of games because they are starting two MVPs and this is a player-driven league. But when the chips are down and it's time to show your cards, I fully expect Westbrook to resort to his bad habits and do more bad than good.