I have a Taranis Qx7 Radio and built an FT Mighty-Mini Scout which flew great, but now I want to put an extra motor and fly it using differential thrust. My understanding is that it needs to be set up on the Mixers Page on the radio, and also I'm not sure how to wire it up.
Some good info at Flite Test with video content as well. If you can create a "mix" in opentx (which takes some getting used to) you can apply the same methodology to setting this up. There are lots of video tutorials etc.. for this. Once you can do mixing on your Taranis, the content below may help.
In the example, they are using a Graupner radio (the same to dislike opentx??), but the idea should be the same. What you are doing is attaching the thrust of the two motors to the rudder channel input so it will change each motor as you yank the rudder.