If I understand the requirements correctly, this should do the trick. Use a different code for "Don't know" in Q2 and a slightly different relevance equation for Q2.

Question 1 (Code Q1) (multiple select question)
1 Answer 1
2 Answer 2
3 Answer 3
DK Don't know (exclusion option)
**Exclusion option:DK


Question2 (Code Q2) (another multiple select question)
1 Answer 1
2 Answer 2
3 Answer 3
DK2 Don't know (exclusion option)
Other, please specify
**Relevance equation: Q1_DK != 'Y'
**Array filter: Q1
**Exclusion option: DK2



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