It's true -- two batteries are better than one. You can connect two batteries to get EITHER more voltage OR more flight time.
You can double your voltage by connecting 2 batteries in series. So, two 3s (11.1 volt) 2200 mAH batteries in series will yield 2200mAH at 6S or 22.2 volts. Note that while you get twice the voltage -- you get no more capacity -- it's still 2200 mAH.
These are two flavors of a series connectors. If you trace the wires, you'll see that both do the same thing.
You can double your capacity (flight time) by connecting 2 batteries parallel so two 3S (11.1 volt) 2200 mAH batteries in parallel will yield a 4400mAH at 3S or 11.1 volts.
Two final notes. 1. To balance the discharge of the individual cells, it's important to always have identical batteries when connecting them in series.
CORRECTION- Parallel cells should be same voltage and closely matched for amperage, but not necessarily identical. (Thanks Doug)
2. Can you spot what's wrong with the schematic diagrams above? (Hint: It's a sex thing)