Behavior to hide the second dropdown in a cascading select

[DanielG]

Apologies - I feel like this has been asked before, but I can't find the answer.

I have a cascading select where on 50% of the options there are no values for the second select/drop-down field and I'd like to hide/disable the field so users can more easily understand that they don't need to provide an additional option.

I understand getting a whole field via getFieldByName, but how do you get a specific sub-field within a cascading select (as opposed to just finding the value?)