Quite a few duets, Lou & Peter Berryman, Loraine & Bennett Hammond, Pint & Dale, to name a few.

Larger groups are out there too. In the "big time" you've got groups like Altan, The Battlefield Band, Steeleye Span have been around for years. Out of the limelight there are small groups (3+ members) all over the place. I sing in a group of 6 myself.