maleien
Try creating two multiple choice question and randomize the choices(include even number of stimuli in each question as choices) in them and hide the two questions.
Post which you can use option wise pairs of each of multiple choice question E.g. Option 1 of question1 and Option1 of question2. It should create random pairs and should accommodate your logic.
Hope it helps!

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maleien ,
You can do this:

1. Add a MC question with all your stimuli. Add advance randomization to only display half of them evenly presented. Add JS to hide and/or click next.


2. On next page, add a MC question and carryforward the non-displayed choices from (1) above. Add JS to save the displayed stimuli to embedded data fields (stimulus1....stimulusN) and click next. (JS shown below)


3. Add a loop & merge block with your two questions. Base the loop on the displayed choices in (1) above. Field 2 will be the embedded data fields set in (2) above (e.g., ${e://Field/stimulus1} where 1 is the loop number).

JS for (2) above:
Qualtrics.SurveyEngine.addOnReady(function() {
  jQuery("#"+this.questionId+" label.SingleAnswer>span").each(function(i) {
  Qualtrics.SurveyEngine.setEmbeddedData("stimulus"+(i+1),jQuery(this).html());
  });
  this.clickNextButton();
});
You'll probably want to define the embedded data fields stimulus1 through stimulusN at the top of the survey flow so are saved in your response data.

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