He should be lucky because Harden relives pressure off of him. It's no longer Lin's duty to perform night in and night out, it's now Harden's duty.
Those two compliment each other well though but it's not like they make each other into very good players. They are both very good as is.
Agreed they both tend to be careless with the ball so having them share ball handling duties instead of making just one be the main ball handler works in favor of the rockets. It also helps that harden is an excellent spot up shooter and that lin is pretty good at getting a pass and driving to the hoop(needs to finish better at the rim though).