Couple of thoughts ...
There have been a number of videos that show the "devastating" effects of the 12ga Bolo round. They all show basically the same thing: at the ranges tested (~30 yards or so) the stupid things don't really separate. The pictures attached are representative.
That said, I don't really think that expanding the concept to a 32lbr. cannon would produce different results. That is to say that the ballistics would be horrid and the damage would be negligibly different from a double shotted load.
It seems to me that the incremental manufacturing cost would be better spent on simply getting more ammunition.
There is an outfit named Advanced Ballistic Concepts that made a round with a three part tethered projectile.
https://www.facebook.com/MI4Bullet/
You fired the thing out of a 12ga barrel that had a rifled extension. The centrifugal force caused the thing to expand. That overcame the objection to the vanilla bolo round projectile.
It would also have worked in a rifled cannon, to your point.
You'd think that the Royal Navy would have had a "committee" or something back in the 18th century to evaluate these things.
My Spidey sense screams "cul de sac".
BTW ... the picture on the right shows the impact of two rounds. You can see that the leftmost impact shows both bolo balls with the same entry hole.