A duet is two people singing or playing musical instruments together. You and your brother might be a popular sidewalk duet with your fiddle-accordion combo.
If you have tickets for a musical duet, you'll be watching and listening to two performers. You can also call the composition that's written for the two of them a duet, since it has parts for two instruments. Two dancers can also perform a duet, a dance especially choreographed for a couple. Duet stems from the Italian duetto, "musical composition for two voices," from duo, or "two."