Constant Sum Custom Validation - Compare 2 constant sum question choices

This can be done by piping the value from the first question into the custom validation of the second question.
For example, let's say your two questions had the following race/ethnicity categories.
Find the QID values string to pipe in the value from each line in Q1. Temporarily pipe it into a place on your survey so you can copy the full string, and paste it into a Word file or something similar. There are a lot of different things available to pipe for each question, so make sure to select the Entry Values, which are further down the list of options, and not question text, descriptions, or choices. The string for Entry Values should look something like this: ${q://QID106/ChoiceNumericEntryValue/1}. Repeat this for each line in Q1.

Then in Q2, set up custom validation for each line, similar to what's shown below (note that it only displays a portion of the actual validation). In each text field circled in red you will paste the piped text string from your Word file for the corresponding row in Q1. After you complete and save, make sure to test out each line of the validation thoroughly.