Here are a few ideas for you...

Option 1: use survey flow to determine which questions will display, and add display logic to every question

• In the Survey Flow, add a randomizer and logic that will determine if a question will be displayed (or not). The randomizer will randomly select 9 items from each set, and set embedded data to “1” (or “yes” or whatever you want) to indicate that item should be displayed. Do this once for each set (so you’ll have two randomizers in the survey flow - one for set A, one for set . Each one should look something like this:

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• Make sure all of your questions are in one block. The randomizers in the survey flow should come BEFORE your question block. (Note: I only put one question in my example block titled “Set A & Set B.” Your block will have all 36 questions in it.)

  

   

• Go back to your builder. Add display logic to every question. The logic should be based on the embedded data you set in the survey flow. It would look something like this:

  

   

• Click on your question block and select “Question randomization” under “Block behavior” on the left hand pane.
  • Select “Advanced randomization” on the screen that pops up.
  • You’ll only need to randomize the order in which questions appear. The display logic you’ve already set will take care of the subsetting, so don’t use the “random subset” option.
  • If you want to limit the number of questions that appear on each page, do that here (since normal page breaks don’t work with block randomization). 

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Option 2: (easier to set up in my opinion, but won’t let you intersperse selected questions from set A and set B)

• Put all questions from set A into one block, and set B into another block.
• Use the block options randomizer to select a random subset of 9 questions from each block. Set your page options here, too - if you only want x number of questions to appear on a single page. 
• In the survey flow, randomize which set appears first (set A or set . This is the most randomization you can get with this method. All questions from set A will appear together (but in a random order), and all questions from set B will appear together (but in a random order).