Well, speaking as half of a duo, I'd make two points:

Firstly, 'more variety' isn't necessarily the same as 'better'. It may well be, and certainly it can be wearying to sit through a lot of songs that all sound exactly the same, but variety is only good if the various sounds are individually pleasing, which is a highly subjective thing.

Secondly, it depends on the duo. We're both solo performers in our own rights, so a typical set can include different voices, accompanied or not on various instruments. Other duos specialise in one particular sound - it might be two voices in harmony, or one voice and one instrument. Then you only have as much variety as a solo performer, based on your choice of material - faster, slower, upbeat, pensive, tragic or comic. A solo performer with a good choice of material can produce a good, varied set, more so than a duo with only one sound and tempo.

Fortunately we've retired from the day jobs on a small pension, and don't need to make a living - just to cover costs.

Andrew
This is us