Whether two people are viewed as a duo, and therefore ‘one act’, at a ‘session’ might also be affected by whether the norm for that session is that everyone joins in with a number if they know it. If that’s the case, the second of the two people, joining in, is just being a normal member of the session even if they are adding something that no one else is, or is capable of, adding. If no-one else normally joins in then the second person is likely to appear to be part of the ‘act’ and the two people will appear to be a duo whether or not they are adding a specific accompaniment, harmonies or just singing in unison.   (I have read that, despite the Copper Family, harmony singing was not usual in tradition English music so perhaps adding harmonies should not be the main criterion.)

That still doesn’t address the question of whether it should always be one song/tune per person or per act but it might go some way to explaining the reasons why, at a particular session, a decision is taken.