I have clutches fly wheels and transfer cases complete or just the gears if you have the casings.
this would be the cheaper way to build a twin set but have no spare fan drives.
if you are offered a twin set make sure it is a tank set up as industrial 6-71 twin sets look similar but transfer case is different ie geared down also if it has come from a quad pack may turn the wrong way, fine if you do not care adout top speed but you would still need to figure out cooling fans unless you have the fan drives and gears for cam shaft to drive them.
if you get offered or see for sale twin sets if you want send me or post pictures will be able to tell if it is industrial or tank set up,as have seen industrial twin set advertised as ww2 tank unit.
i do have a couple of twin sets tank units complete and know of 3 others that are the owners may sell if offered the right money to part with them.
alao have a industrial twin set if you want to go slower and cheaper!