How to power two stacked motor shields with one 3S Lipo battery (11.1V)? I have these 2 motor shields:
Adafruit Motor Shield V2 and 
Seed Motor Shield V2.

They are stacked on top of an Arduino UNO.

My question is, how can I power 2 of those motor shields with one 3S Lipo battery(Turnigy 3S 2200mAh 20C), given that I have 6 identical 4.6-6V DC gearmotors?

Thanks!
You can avoid using a linear voltage regulator. Try using this voltage regulator.

You will get 5 Volts 2 Amp output which should be easy to connect to motor shield's input power ports.