Disney still has the rights to Shippuden according to Jason DeMarco. They can't get it until Disney's contract is up.
They are paying for Naruto with each episode they run. That's why they can't double up either. At the same time, Naruto's been highly requested so they're gonna give Naruto a shot to see if it can pull down some ratings.
Also, it's a show that will be left untouched by standards and practices and they want to show the uncut footage from the very start. The hope is by starting over they can win over newbies as well as pull in those who've lapsed.
I am sure AS will double up once Shippuden becomes available and if Naruto's ratings are strong.