More barrels are not always better, as this double barrel cannon built during the Civil War illustrated:
This cannon currently sits in downtown Athens, GA (home of UGA, not known for mechanical engineering training), where it was developed by a local businessman.
The idea was to connect two cannon balls with a chain and mow the enemy down like a scythe cuts wheat.
The test firing, while declared an "unqualified success" by a local paper, had the following results:
The Cannon was taken out on the Newton Bridge Road in April 1862, for test firing. The test was, to say the least, spectacular if unsuccessful.
According to reports one ball left the muzzle before the other and the two balls pursued an erratic circular course plowing up an acre of ground, destroying a corn field and mowing down some saplings before the chain broke.
The balls then adopted separate courses, one killing a cow and the other demolishing the chimney on a log cabin. The observers scattered in fear of their lives.
Some reports claimed two or three spectators were killed by the firing. The reports of the deaths [what we'd call "freindly fire" today] have not been substantiated.
The idea was not new. Antonio Petrini invented a double barrel cannon in 1642.
Read more about it here, if interested.
Sounds like it had potential as a minefield clearing device, but I don't remember reading about extensive use of land mines during that war.