Keith, I have to eat my words, I "misremembered". Both Greener and Taylor spoke rather well of that design. Greener said "Whenever large game is occasionally to be met with, they form the best armament for the sportsman." Italics are Greener's. He said they shoot as well as the common cylinder bore but not so well as a properly choked gun.
Taylor said "The Paradox was originally brought out by Holland & Holland, but I think any gunsmith will build you such a weapon nowadays". " They mostly have their own names for them such as Westley Richards "Explora" and somebody else's "Jungle Gun". Apparently the names were patented but the concept was not. Taylor went on to say "I can strongly recommend one of these weapons, they are most useful and a handy thing to have, loaded, in case of an attack on the camp at night". Taylor liked it for leopard with a 750 grain soft lead slug in the rifled barrel and SSG buckshot in the other.
They both speak of side by side doubles and that is where I got the notion they were built only that way.
The closest I've come to that concept is a rifled choke tube in a smoothbore Winchester pump. The pattern with birdshot was pretty useless at 25 yards and the accuracy with slugs was no better than with the improved cylinder tube.
There is good reason for them to be heavy though, loads are listed as 750 grain bullet and 4 1/2 to 5 drams of black powder or Cordite equivalent. That would be 123-137 grains of powder with 1.7 ounces of lead and firing that from a 7 1/2 pound gun would be flat painful.