Arizer Air Dual Battery Charger
To take full advantage of two batteries, Arizer has this nifty two battery tandem charger.
The dual battery charger can charge 2 - 18650 batteries at the same time. It has built in overcharge protection for any protected batteries that you use to charge your other mods.
Naturally you can charge one battery at a time as well. This way you can use your Air while getting the other battery fully charged.