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.
, But my problem occurred while manoeuvring in a marina, apart from engine starting I had not used the lower throttle and it was in neutral when we left, I can only assume that during my complex manoeuvres that involved turning the boat before berthing the controls clicked together and the lower one remained in ahead until I noticed the problem.
I am now careful to check the lower control when approaching our berth.
Thanks to all those who have commented on this problem.