Driving Sabertooth 2x12 in INDEPENDENT mode

Hello, I have been looking all over the internet for some help with this but I have not found anything. I am trying to run the sabertooth 2x12 in independent mode, which according to the manual, means that the signal fed into S1 is controlling motor 1 and the signal fed into S2 is controlling motor 2. I have actually written a code where I can control both motors 1 and 2 using only signal S1. I am confused as to how this is possible in independent mode because according to the manual it should only be controlling 1 of the motors..

MY question is, how do I set up having the two signals? Where do I plug them in to? I currently have signal S1 going to the tx1 port on the arduino. I have only seen this set up online and nobody talks about where to hook up signal S2 or what to do with it if you want to run in independent mode.

Any help would be great. Thanks

From the manual that I found here:

Mixing Mode: With switch 1 in the UP position, the controller is in mixed mode. In this mode the signal sent to the FWD channel controls the forward/back motion of both motors, while the Turn channel controls the difference in speed between the two motors. Switch 1 in the DOWN position (shown) allows control of the motors independently.

Emphasis mine.

Fwd/CH1 connector – This connector controls the forward/backwards motion of your robot in Mixed mode, and Motor 1 in independent mode. It is usually connected to the elevator channel on an R/C receiver. Turn/CH2 connector - This connector controls the turning motion of your robot in Mixed mode, and Motor 2 in independent mode. It is usually connected to the aileron channel on an R/C receiver.

A more complete manual.