There are a few companies that do tandem conversions around here Usually when they do them they use cut down duce and a half military axles because of the pinion locations. Incedentilly the 2 rigs pictured are probably the two most popular 6x6 pickups and they are made by the same guys. The two pickups pictured above came about because of the T-Rex concept truck that dodge made before the germans came in and ruined the company. The concept truck proved to be popular with their fanbase, but was deemed too heavy for consumers by the germans. The T-Rex concept truck achieved the 6x6 by using a dual transfer case conversion and standard rear axles.
No had a buddy who was a welder on the oil pipelines and put a helper axle on the back of his welding truck didnt even get a second look when he went to inspection every year.
you could get crazy custom on a gm 10.5. they have a third pinion bearing design like the ford 9". you could weld a shaft on to the back of the pinion that will exit out a hole you would put into the cover. then put a yoke on it and then you could attach a drive shaft to it. now this would be way more expensive and much weaker than using diffs that are already designed for this. now maybe do a rat semi truck type build.
That's kinda what we had sketched up. Tbucket look with a wrecker hook on the back. Became less function and more show. Original design was an old led sled.