I have to assume that there was as second banjo player. There are a large number of sheet music covers of minstrel groups with two banjos, even in the 1840s. It's unlikely that the second player was up to Briggs' standard. The second player could possibly have doubled on other instruments, like Emmett did.
There are probably many ways we could conceive of the role of a "second banjoist" in the early minstrel show. The "banjo solo," was, of course, a staple, and obviously Briggs' role; perhaps the second banjoist played in ensemble numbers.
Some of the 2nd banjo's role might have been the sort of chord/arpeggio accompaniment stuff we see in the song arrangements in the Buckley (and other) books. That could be applied to tunes as well.