"Traveller's Pistol" says it all.
Even in the days of old, thugs were often encountered in pairs just as they work now.
Having a big bore double pistol in your hand must have been quite a deterrent!

I really don't know what makes a double pistol a "howdah" or not.
I think as muzzle loaders, they would be of the same bore as their shoulder fired companions, for the simple reason of simplified reloading in the heat of the hunt.
Also, I have noticed that most muzzle loading howdah pistols have their ramrods affixed to the barrels in one way or another to avoid losing them since they were often simply hooked on the belt and not holstered.