Here's an idea. Take this 4-motor controller for $25 and connect two of the drivers in PARALLEL. That means you tie the motor outputs together (+=>+) and (-=>-) and connect the two DIR inputs together and the two PWM inputs together so ch-1 & 2 drives
motor -1 and ch-3 & 4 drives motor-2. The current is split between the two drivers per motor so the heat is spread out automatically. The driver inputs don't know they are tied to the other inputs and the motor current is split between two drivers for each motor. Another option is you could buy TWO of the first one I recommended (the 30A) because you can drive a smaller motor than the driver was made for without damage to the driver or the motor.