Seconding
The term "seconding" refers to the practice of adding a harmonic/rhythmic accompaniment to a fiddle tune. In modern times, this is most often provided by someone playing the chord changes on a guitar, but in earlier days (when the pool of players in a small town or village might be limited), the accompaniment might be played on a banjo, mandolin, piano, or even another fiddle.
Bob Holt (fiddle), Alvie Dooms (guitar)
Skillet Licking
You might come across the term "skillet licking" in your research on old-time fiddle music...it refers to the seconding practice of playing a lower harmony part on one fiddle while the other fiddler plays the melody. The term references a Georgian group from the 1920s-1930s, called Gid Tanner and the Skillet Lickers, who added a third fiddler to the standard string band instrumentation (a practice that had been common at square dances and house parties long before the Skillet Lickers).
Fiddlesticks
"Fiddlesticking" is another way to add an accompaniment to a fiddle melody, even when there is no second instrument handy! All it requires is a pair of thin sticks or knitting needles, and a lot of finesse!