If I’m understanding it correctly. you want to show 2 different answer lists for two different respondents in same set of questions. You can create randomizer under survey flow and create one embedded field. under the randomizer add two embedded field module for the same embedded field and set the different values into it as mentioned in below screen shot.

 

Then merge both answer list and create one hidden question (hidden with JS and auto forward), insert that merged list into it and randomize the choice of that question. Create this question before the loop and after the randomizer punch in survey flow. Apply the display logi (AnswerList=1 for first list and AnswerList=2 for second list) on each answer option as mentioned below. Then your list will appear based on randomizer value.

Then you can select loop and merge question and carryforward as Displayed Choices of the hidden question’s answer list in that question or even outside the loop (anywhere). refer below screenshot. Try it, it will work.
 

I want to implement the following,

ListA: a, b, c

ListB:1, 2, 3

The first question text reads;

“The first letter is (ListA), number (ListB)”

The next question reads;

“The second letter is (ListA), number (ListB)”

The last question reads;

“The third letter is (ListA), number (ListB)”

Where a letter is randomly selected from ListA and ListB each time and each question appears on a separate page, with a 45 second time limit for the entire block. 

Ok then first create 6 embedded fields.

Then use 6 different randomizers in survey flow. 3 for letters and 3 for numbers.

Below is for letters, create same as for numbers and punch value as 1, 2 and 3 instead of A, B and C.

The pipe in Letter1 and Number1 in first question, Letter2 and Number2 in second and Letter3 and Number3 in third question text.

I understand. Thanks. I’m disappointed qualtrics doesn’t have a way to randomize each field in loop and merge, it would make a lot of tasks in behavioral economics studies a lot easier. Also, There should be more interaction with excell and qualtrics. 

How may questions you want to ask and how many letters and numbers are available in your List A and List B?

Here’s a demo of the stroop test: https://demo.survey-awesome.com/stroop