Dang, i just spent some time looking at my tranny case's and adapter. You got my mind rollin 2many, thanks
So in you second pic you 2wd/4wd stick is a top shift.
What i have come up with in my head is a:
forward shift high low for the front case,
top shift high low for the rear case,
and forward shift 2wd/4wd from the rear case through the adapter to the front case top shift hole.
dang, did that make sense, it would be your poor man's dual case twin. damn, will my truck even accept 4 shifters in a row 2 feet long.