I drove many thousands of miles in an old Peterbilt that had 2 transmissions. It was a popular way to gain lots of close-together gears so you could always find one just right for the speed, load, grade, etc.
It had a Spicer main box that was on the back of the engine, like any other transmission. It had 5 forward speeds plus reverse. Off the back of that was a short drive shaft that led to the font of the auxiliary transmission with 4 speeds (no reverse). So you had a total of 20 forward gears (gear combos, really) and 4 reverse.
Shifting that rig was quite an art. Not for the faint of heart. Looked something like this, when you got the knack of it:
https://www.google.com/url?sa=t&rct=...whbNQ7IMa-IRdj