Grew up driving trucks with 4-5 speeds w/ 2 speed's AND 3-4 speed trans behind the 4-5 speeds. I was driving them at 16 yoa.  The aux. 3 &4's weren't scnro'ed. Used to grow allot of potatoes in the area, required trucks to 'craw' along at a walk.

DBH is correct, put the aux. in 1st in the field and use the 'main' trans. for shifting gears.

Once out on asphalt, stop and shift 3/4 aux into direct/over or what ever. The idea is being able to slow the truck down enough in the fields with enough engine rpm's to not lug the engine to death. Just because there's 2-3 seperate transmissions in a  truck,, DOES NOT mean you have to be a little kid playing with sticks.  And, yes after getting experienced I could easily reach thru the steering wheel and shift both transmissions at the same time. (didn't mean it was necessary)

Drove 2 trans. spud trucks for years in fields so steep the spud harvester's could ONLY dig going up hill. Digging down hill the spuds would just roll back down the primary digger chains.  I had to drive uphill along side the digger, then BACK down the hill pickup the 2nd digger and load while digging up hill, repeat.   LOL... 5 "truck drivers" quit the first 3 hours we started digging, and walked 5 miles back to the shop to get their vehicles. They seemed to think the fields were "too steep".  LOL...  (weenie flatlanders..)

Just because there's 60+ 'possible' trans combinations does not mean you have to use every one of them every time a person shifts gears.