There were several types of "TWO STICK" transmissions. After the Maxidyne engine came out, with the 5 speed transmission, Mack also made what was known as an extended range transmission, that did have 2 gear shifts, but is not anywhere near the same as the old original 2 stick transmissions. The Duplex was essentially a 10 speed tranny with a 5 speed main box, and a 2 speed auxilliary. For each gear on the main box, you split it with the auxilliary transmission. So each MB, gear was divided into 2 steps. The triplex had a 3 speed auxilliary, and a quadraplex had a 4 speed auxilliary. With the Quadraplex, when you upshifted the main, you brought the aux immediately back to 2nd, if you brought it back to low/1st then your speed ratio was the same as the previous MB gear in high on the aux. THIS CAME IN HANDY WHEN YOU WERE PULLING A HEAVY LOAD UP A HILL FROM A START AT THE BOTTOM. For the "most part" unless you were really havily loaded, you started off in 2nd on the MB, and 1st on the Aux. You progressed through the aux to 4th, and then the next shift, you shifted to 3rd on the MB, and then brought the aux immediately back to 2nd. Then you upshifted to 3rd aux, and then 4th aux, and then the MB to 4th, and the aux back to 2nd. Down shifting was the opposite. If you were really havily loaded or in soft dirt, you started off in 1st MB, and 1st in the aux. (being really easy on the clutch to keep from throwing a U=joint, or wrinkling the drive line.) There are some videos on youtube if you want to see how the 2 stick was progressively shifted. I also created a visual video demonstrating how to shift a quadraplex. these types of transmission were primarily designed for the old gasoline and early thermodyne engines, with low torque factors. If you try to use one behind a maxidyne or later variant engines, it will take out the bearings in the trans... pretty quickly, and I have heard it will break the gears, so I wouldn't recommend using one with the high torque engines. I hope this helps you. Dan.