There is no easy way to do dual cases.
Rob G, Troy, and myself figured out a way to use an isuzu auto case behind the stock case. Its a roll your own deal and much fabrication is involved. We all went with slightly different methods to achieve the same results. Basically you cut off the front output portion of the stock case and mount up the auto case behind it. Much more to it than that, but thats it in a nutshell.
Corbin found that with the bellhousing from a '98+ trooper 5speed he could use a toyota turbo tranny and then used toyota dual cases. Randii, back in the early days had an ultra rare one off bellhousing that mated the 2.6 to a toyota tranny with dual toy cases. Dan pitched the isuzu tranny and went with some other tranny/tcase combo. Dallas swapped a 4.3/700r4/dana 300 combo.
So, there are many ways to get there, but none are very easy.
I always really liked Corbins set-up, but it would be expensive to replicate.