A very cheap possibility which would be easy to do. I WON'T comment on the legalities, anyone trying it would need to work it out for themself.

Take an older double shotgun. Shorten the barrels and the stock. Use two of these .45/70 (or other calibre) insert tubes. These would need to be docked as well and fitted.

Voile!

Would this work? I don't know, but the barrel length, bullet weight and velocity would be important to get the bullets to stabilize.

Now does such a setup sound accurate? Probably not, but I don't think Howdahs were very accurate in the first place, so nothing new. My reading was their main purpose was a defensive weapon when the tiger one is hunting is hanging onto the head of one's elephant and one's rifle barrels were too long

for one