No bother, we'll get you squared up & running smoothly.
Sounds like you need to adjust "center" for aux 2 so both tracks travel equal speed, same direction when moving Aux 3 for/rev.
Go to menu - servo adjustment - (travel will appear, click it) scroll to sub trim to fine tune center of Aux 2.
Also under servo adjustment, you may want to adjust turning speed of Aux 2, being that the 3pos switch is +100% > 0% > -100%. Go back to Travel to lower + - for both directions for slower control... but much less than +-100 and the tracks won't spin opposite each other when Aux 3 is engage, rather one track will just slow down. Your preference how you'd like it to work.
I was hoping to get my 2x25 Sabertooth in my JD850 to work just like your's is now, but it didn't... I had to incorporate the flight mode switch & jiggle with the subtrim & travel of both Aux 1 & 3... it's so complicated I can't remember how I did it.
So be real careful to never short out that dual esc you have b/c they're extremely pricy to replace.
