Hi there, I think the validation is not working because the sliders are not actually empty, but are "15". I think the Constant Sum question will always have a value selected and is why the "Force Response" option is not available for Response Requirements.
To get around this, you can use the Slider question type and set the sliders to "Force Response".
You can also change the Custom Validation in the Constant Sum sliders so that it is based on something else. You might try making it so that both sliders are not equal to "15" or add a Timing question and make it so that the click count on the page must be at least 2.
Another option would be to use JavaScript to add a fake next button which will only advance the page if both tracks have been clicked on.
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