Transistor setup for large motor circuit
Do not put the whole shebang in series. Instead deploy your circuit with each battery individually connected to a motor and then through a N-FET to GND. PWM Drive on the N-FET gates will allow you to control the speed of the motors. Use two PWM signals to permit individual control of each motor's speed.

Selection of an N-FET type will take some careful work. Since your motor can draw 27A at normal load (133A at stall) you will need some beefy parts. If you design the circuit around nominal conditions plus a guard band you may choose a 70 or 80A N-FET. If so then you will have to design the circuit with some intrinsic current limiting or add some current overload sensing that couples back into the FET gates to shut them down when the current starts to get to high. Further details regarding FET selection are stymied by lack data regarding what you want to control the gates of the FET with.

Keep in mind that this configuration causes you to only be able to control the motor in one direction of rotation. If you require bi-directional control then you will need an H-Bridge for each motor that consists of four power FETs per bridge.