You have to trigger the field in this way:
Go on Settings -> document properties -> triggers
in the field event triggers box select your field and add extion -> external -> set variable
write in the upper box vShow and in the lower box write 1
Now in the script you need to define the varible in this way: LET vShow=0; and reload your document.
The last thing, the secon list box must be conditioned to be shown only if vShow=1 so go in layout -> conditional and write vShow=1
Let me know