my ESV has a switch where I can read 6 volts or 4.8.......
Popriv.....6V will give you more torque and speed from your servos......for more flight time you want a bigger capacity battery say a 2400 mAh.....
for 2 batteries connected to a receiver  they would be in parallel.....where voltage remain constant and current (mA) is additive....the same as a bigger capacity battery