The original unit came with a lever to clip on the handlebar. The top plate of the lever was marked High, N, Low to show which way the operator had to move it after pulling in the clutch lever. It was a rather inelegant solution as the rider had to use their left hand to pull in the clutch lever, their right hand to operate the twistgrip and the other hand to move the lever. You get the picture.
It's not as bad as it sounds. I've ridden a 1929 two speed Excelsior two-stroke for many years and you soon learn how to do it without cocking it up. You pay MUCH more attention to your riding even though you look slow to bystanders.
Cheers,