Obviously it depends on the person MCing but if I were doing so (which I don't) it would probably depend on how many people were vying to perform. If I was going to struggle to get everyone in then a duo might get one song or tune, with the possibility of being brought back at the end of the evening if there's a gap.

On the other hand where I spend my folk time we are pleased for all comers and even on relatively busy nights every person is asked to perform... if he wants to include someone else in his performance that's up to him. Generally if A accompanies B then A will be expected to lead the second song/tune but if B is the singer and not A then no one's going to complain if B sings twice.