RE: Batteries in a series - C's
He showed you where the 108 amps came from. 20C * 5400mah / 1000 = 108 amps.
When you put two identical batteries in series you do NOT double the mah available, you double the voltage present. In your case with two 11.1volt 5400mah batteries in series, you would have in essence one 22.2volt 5400mah battery. The C rating also would not change.
But then, isn't that what BarracudaHockey said?