A 2S lipo fresh off charger will be 8.4v. Emaxx esc effectively wires the two batts in series so you'll have 16.8v from 2 fully charged 2S lipos. Lipos voltages are talked about in two ways - nominal voltage is 3.7v per cell - 2cells (2S) is 7.4v nominal. Fully charged, lipo cell voltage is 4.2v, so this 2S lipo is 8.4v charged. Minimum voltage is 3v/cell or 6v for a 2S lipo.

If you tried 2 charged 3S lipos, you'd burn your esc out with 25.2volts. 3S charged is 12.6v (3*4.2v/c).

Get the 2 - 7.4v lipos and charge them - they'll do what you want.