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If I'm understanding the question correctly, then I think you want to merge the fields together or create a new field for the join that implements the logic you outlined. In either case, you'd do this prior to the join.
If your fields are named p1 and p2, you could create a calculated field named Join with code like this:
That would give you the value of p1, unless it was NULL in which case you'd get the value for p2. Then you can join on that field.
Alternately, simply select both p1 and p2 (CTRL + click) in the flow before the join and select Merge. It implements the same basic logic.
Hope that helps!