Is it not possible to build a sublist or create a derived answer based on comparing the answer in one dropdown field (in my case in a grid) to the answer in another dropdown field (in my case in the Q&A table), where the two fields have the same options in their respective dropdowns?

from Noam Raz

Thanks for the question.

We'll need to interpret using a couple of technical terms so we can try for an accurate answer. Assuming that you're asking about a Question in the Questionnaire and a Row in a GRID that have the same Answers, we'd want to delete one of them. As a firm rule, we really don't want the same Answers in two different locations because it may (will) result in chaos in the form.

If the goal is to combine two independent answers, yes you can use a Derived Answer with a Series Answer or a List composed of the Answers from each, even nested Answers if need be. There are a number of comparisons possible (larger, later, various math answers). 

There may be others depending on your goals. Almost anything you can accomplish in the Form can be accompished in a Derived Answer.