Connecting in series does NOT add up amp hours.

If you had two 2200 mAh 11.1 volt packs, you could either connect them in SERIES, in which case you get 22.2 volts, but 2200 mAh.

Or, you can connect them in parrallel and get 11.1 volts, but 4400 mAh. Both voltage and amp hours don't add up together, it's one or the other.

If you had 4 2200 mAh lithium batteries, then you could make two packs that are both 11.1V 4400 mAh, then connect all 4 together to make them 22.2 volts, 4400 mAh. But now you're talking about charging 4 seperate batteries for a single flight, that's alot of work.

2200 mAh batteries, even if they were rated for true 20C discharge, would not last long on the t-rex 600. You'd end up killing the batteries since amp draw is simply too high. The helicopter hovers around 30 to 40 amps if I recall correctly, which is already nearly exceeding the batteries discharge rating, as soon as you add quick pitch changes, you're peaking 70 amps or more.

4400 mAh, really, is the bare minimum, you should actually go HIGHER. More weight, but your batteries will atleast last longer.