How so on the latter? I guess I don't really understand. What would be the advantage of one setup over the other?
Well, one way would be to disconnect the switch inside the t-case, and simply replace the segment of circuit with a manual switch. Now the driver does to the ADD what the t-case used to do - and needs to remember to shift the lever into 4WD before
hitting the switch if doing it at speed (or you get a real nasty slam in the front diff). Also you want not to hit the switch accidentally, if at speed.
The second way would be to interrupt ONE of the wires going to the t-case, and add a manual switch in series with the one in the tcase. This way you could hit the switch accidentally or out of sequence while at speed, and nothing will happen until you shift the lever, when it will behave as normal. Both switches need to be closed to activate the ADD. When in 4WD, you can operate the manual switch to disconnect the front end, to allow 2WD low range (or just to stir up the diff oil on pavement).
I'm pretty sure this will work, except it was hard to get my fingers and the strippers at both ends of the cut wire, so I ended up doing it the first way myself.