How many instruments do you need to make a band? Not many. Lester Flatt once said that in the old days, "Somebody'd mention a band, and one of 'em would reach and get a fiddle, and the other a banjo, and they'd have it all ready to go." And a banjo goes not only with a fiddle, but also with a mandolin. Murphy and I explored that possibility last Saturday by playing a party gig with just the two of us, on mandolin and banjo.
How do you keep a good sound with just two people, and both of them playing lead instruments? For one thing, you play as solidly and steadily as you can when you're playing lead. For another, when you're playing backup, you LISTEN to the other instrument and play what sounds best for backup. Most of the time it's pretty simple backup, always keeping in mind that you need to be as supportive and non-distracting as possible when the other person is playing lead.
At the gig, Murphy and I played a pretty full gamut of bluegrass instrumentals, including quite a few of the old tunes we used to do as fiddle-and-banjo duets (Old Joe Clark, Sally Goodwin, Little Rabbit, and so forth). It all worked fine, and in spite of our being just background music at the party, we often got applause after we'd play a number.
The key to a good sound is LISTENING, whether you have five people in the band or just two. If you have your music together and use your ears, you can play a job quite well with banjo and mandolin!