A toyota gear drive case has two shift rods that are actuated by the shift lever, in stock form you are only allowed to operate one at a time (using an detent inside the t-case)
shown in the bottom right of this picture
a twin stick shifter actuates these shift rods individually, allowing one shifter to control 2WD-4WD and the other to control HIGH-LOW, thus allowing you to have 2WD low. Plus more shifters in addition to the dual cases 8)
you do not need dual cases for the mod, but you do need to remove parts of the transfercase to allow the shift rods to move independently. find install directions online and you will see the work involved.
-Zach
