I believe I was also told this for my 321.
I assumed it is only common sense because it is basically saying ‘do not have both throttle controls open at the same time’. I thought that if you leave one open and then use the second one you will not have full control over being able to both close the throttle and put the engine into neutral.
I am sure someone else with better knowledge of the mechanical situation of the throttle controls will be able to give a better answer.