The simplest method would be to just have two motors, with one motor driving each side of track.
To go forward, both motors forward; backward both back; to turn set one motor to go faster than the other, only drive one side, or set one forward and the other backward. It is a fairly common treaded vehicle steering system.
The linked system with dual differentials, a drive motor, and a control motor, seems quite excessive to me.
However, it come with some cost as the system move without feedback, it's unlikely that both motors will be driven at the exact same speed. Variations in the build (e.g. how tight bushings are) and the motors (torque curves) means that it'll generally drive in a (large) circle unless you actively correct for it. To make it move relatively straight, it involves a lot of fiddling with loosening / tightening parts, and even then one may often find that it no longer drives straight on a different surface.
With a dual-differential drive, there's generally enough friction that it will indeed drive "exactly" straight when the drive motor is on - and if it doesn't, one can just add a bit of friction (e.g. clamped bushings around a stud) to the control motor to make it drive straight.
