When I ran a club I reached the conclusion that using the number of songs as a measure was not useful for good time keeping. Obviously, some people had very short songs and some the opposite. Also, some people would add to their time on stage with eg, tuning, jokes, banter, starting again because they had made a mistake. And what about instrumentalists who could string four or five tunes together and call it an item?

My solution was to ask people to think of their slot as 12 minutes. They could perform as many songs or tunes or jokes or indulge in banter as much as they liked as long as they kept within the 12 minutes. This tended to make people consider the slot as a whole entity rather than just two songs. Generally speaking, it worked quite well. It doesn't really address the discrimination against duos issue but I did apply it to all of the acts.