I guess you're right the dual range can be shifted on the fly. paraphrase from a road test "The dual range gearbox takes some getting used to. Most testers ...left it in low range until they got to high gear, then kicked it into high range for cruising". But i guess you can play around with it all you want.
I think in general you would leave it in low range for city work and high range for highway. Not continually changing ranges.
Kinda like hi/lo range 4WD vehicles.
You can't make a mistake. whatever gear you are in say 3rd, you can go hi range 3rd or low range 3rd. Then you'd have to change to 4th in the same range you were in. Once in fourth you can change ranges, and so forth.