Firing two balls and the chain out of a single barrel limits the length of chain. That's okay for tearing up ship rigging but to be really effective against a formation of troops a longer chain is desirable.
The Confederacy tried a double barrel cannon to fire two cannon balls tethered together with a substantial length of chain as an anti-personal device. Problem was if the barrels didn't fire exactly at the same time the first canon ball would wipe around and kill the crew. Idea abandoned. One of those canons graces the front yard of the Clark County Courthouse in Athens, GA (U of GA).
I'm thinking a really long barrel with two very large hollow based bullets back to back. The chain would be held inside the bases. The total mass of the assembly would be about the same as a single shot of the same size.After so many yards the two halves would start to separate, stretching out the chain. Probably a lousy trajectory because of the huge surface area once dispersed though.