The outputs of the TB6612FNG can be paralleled. We use them that way in our Orangutan SVP robot controllers. You might find the motor drive section of the Orangutan SVP schematic diagram helpful. Please note, the 37D gearmotors can draw up to 5A during stall at 12V, which is close to the limitations of what the TB6612FNG can do with its outputs paralleled. However, it sounds like you are have a light load on the motors, so it might work fine.