It would be great to double up the capacity but... we also need to address the durability of the batteries that are without cooling in warmer climates. Even on a 2015 battery in Arizona, we are only getting maybe 3.5 years to a drop of 4 bars.
Sure, if you have double battery a car can degrade a bit more before only being able to go 40 miles, but still sitting or moving there are those constant calendar losses especially from heat. If I could only go 5 to 7 years before complete battery replacement with a drop to 4 bars, it would cost $13,000 to start fresh again with a double battery. Is that worth it?
A double battery will only be worth the added expense if both batteries can be cooled IMO. Which also brings me to the 30kWh battery pack. It needs to be cooled or it needs have a better chemistry than the lizard. We don't know if the chemistry is better for heat. The lizard is not proving better in the heat of Arizona.
*2011 Leaf 1 bought 2/28/15 @ 28,000ish mi 10 bar (8 bars @ 11/25/15 @ 37,453 ) (New lizard @ 39,275 mi @ 1/20/2016) Now 52,166 mi.
*Tesla Model S 61,000 mi
*2011 Leaf 2 bought 4/28/15 @ 24,000ish mi 12 bar (new lizard Dec. 2014 @ 22,273 mi) Now 35,485 mi