I do stuff 'solo' and I'm also a member of a three-piece acoustic band, so I can truthfully say that I don't have an axe to grind from either POV.

My personal feeling is that a solo performer is one 'act', a duo is one 'act', a trio is one 'act', and any other number, up to and including a full choir, is one 'act', and that the only equitable way is for each 'act' to get the same number of songs.

So, if it's two songs (my personal preference), each 'act' gets two songs. If one song, each 'act' gets one song.

A singaround club I used to drive 35 miles to on a Tuesday night had a rule of one song per round. There were a group of four or five members who each did a solo song, then did songs in varying permutations of two, three, or four members, to a point where they virtually dominated the entire evening, and others (including me) probably only got one song.

Needless to say, I don't go there any more.