If you're doing a follow-up, you usually join the data from both waves (using the opt in ID/SERIAL), which would include the REF.
If you want is "more separate", you could show different opt-in questions, depending on the REF in questionnaire one, and assign each a separate subgroup that would appear in the address list, and can be copied to the dataset, using panelData() and put() in questionnaire two.
However, using the REF from the merge from both waves, seems the straightforward solution to me. Give it a try.