Well, no attachment to look at, but a few pointers can still be had
Instead of darlington transistors, use mosfets. They are still a transistor, but a different type.
To have speed control, use PWM to drive the mosfets.
Using PWM will give you speed control, but without any type of feedback circuit it will be difficult to know HOW fast, and trying to match speeds to a second motor will require human input on a joystick for correction inputs.
Just taking a motor and PWMing it down to 20rpm if it is not designed for it can lead to poor performance and max current issues. Using a geared motor that stays within operating limits is a good idea.
The back EMF diode just need to be within current/voltage tolerances.
Try to post a diagram (hand-drawn or other) of the entire circuit you are looking at. While there are only 2 ways to do it (right or wrong) there are countless ways to achieve either. We cannot guess what and how you have it connected up as. Include part numbers.