Double headers sound like a great idea, but in reality its 5 hours long with at least an hour between the games. Sure you can leave the stadium and come back but even then it's a long expensive day!
At a place like Mt Smart, you could have the second game warm-up on peripheral fields, but that would increase maintenance costs at groundsman would need to keep the turfs nearly identical.
Likewise, if you could bring your own food (or the stadium stuff wasn't exorbitant) families might be lured into a picnic at Mt Smart, especially with the big slide. Kicking the first game off at 10am and the second off at midday would be pretty cool but broadcasters would have a fit.
Heaven forbid the beers were $5 a pop... you'd get half of Auckland there!