2 DC motors connected in parallel to H-bridge move with different speed
DC Motors will always run at different speeds.

If you have two motors in parallel on one driver and are only sensing speed from one of them then the other motor's speed will be unpredictable if and when the motors are not mechanically connected. There is no real way to correct that using electronics without adding separate drivers and feedback for each motor.

If the motors are driving wheels that touch the ground and the torque difference between them is not great enough to cause the "tires" to slip this may not be a problem since the contact with ground will act as a mechanical coupling between the motors. Note: That is how the drive wheels on your car stay at the same speed despite going through a differential gearbox. Loss of ground contact can cause some odd effects though.

Otherwise it is prudent to run a timing belt, or other mechanical linkage,  between the motor shafts to keep them together. If your crawler has tracks this is a non-issue.

In some cases it may be possible to share the driver over both motors. That is, have a switching arrangement that channels power to each motor a percentage of the time, giving more percentage to the slower motor. However, that would of course reduce the performance of each motor by 50% when both are at speed. But that gets complicated and does not buy you much compared to having individual drive to all four motors.