While you can purchase a dual shifting unit from Novak for your Dana 20, it's also possible to fashion one from scratch.  Note that I've never used the dual shifter on a Dana 20 transfercase without removing the center pins that move between the shift rods in the the shifting tower.  To remove the pins, you have to remove the shift levers, drill out the plugs on the sides (note where the red gasket sealant is on the side of the shift tower, and then slip out the oblong pins (they are elongated metal balls).  With these removed, you can push and pull the levers all you want (but don't try running high range in the front and low range in the back unless you feel the need to break something).