Well yeah I opened it, it works but not when I insert it in my code, but I'm sure it's about onctrol.
I have found another solution!

Add knob, speed LFO knob. When you select 'not sync button' (position 1), the knob has range 0 to 50.
Until here that's the same as before.
But if I insert this condition:

inline function onBUTTONControl(component, value) { KNOBHERTZ.set("visible", value); }; BUTTON.setControlCallback(onBUTTONControl);

And then add a second knob, with value 0 to 1 for synchro, then add this similar line:

inline function onBUTTONControl(component, value) { KNOBSYNC.set("visible", !value); //Inverse value of the other knob }; BUTTON.setControlCallback(onBUTTONControl);

And then it switches btw the two knobs, but we don't see it! :D

I go try it, I hope it works :P