Handling of deployable conditional unfolding – Exchange of user experience stack

Correct me if I'm wrong, but my understanding of the question is that you do not want the second drop-down menu to appear only if you select a specific item from the first drop-down box.

What you can do is disable the second drop-down menu (and possibly smaller or indented), with the label that explains it.

In the example the first drop-down can have options Little, Medium, Big, Personalized, then the second drop-down is only unlocked when Personalized is selected

Example