If your truck is not a 4 wheel drive, then most auxiliary transmissions will split the gear ratios in your main transmission. If you place the main transmission in low, you can start off in each of the gears available in your auxiliary to get a feel for what it does. If it is a two speed as you have indicated, (3 positions, one being a neutral) your shift pattern could be something like 1L 1H 2L 2H 3L 3H 4L 4H. with the L meaning low gear in the aux box and the H meaning high gear in the aux box. This also would mean every other shift you would be moving two levers at the same time. Keep in mind that it is ok to skip gears based on your load and speed so you definitely don't have to use every combination every time you start from a stop. It is also possible that the aux box you have acts like a low range and is not normally used in highway driving much like the low range found in many 4x4 boxes. One way to tell would be if the gear ratios are relatively close together or are far apart in the aux box.
As an example, my truck has a 4 speed and a 3 speed auxiliary. The auxiliary provides a slight under drive (U), a direct drive (D) and and a slight over drive (O) to each gear of the main 4 speed transmission. My shift pattern is 1U 1D 1O 2U 2D 2O 3U 3D 3O 4U 4D 4O, but frequently I only use 2U 3U 4U 4D and 4O as my usual take off from a stoplight. For a reference of driving with two sticks the videos in my signature will show you what its like.
