A pair of muzzle loading double eleven bore rifles eh?
My oh my...

It is possible that they were set up for conical bullets.
I was talking to Lewis Drake one day recently about loading for the sixteen bore double rifle. He had been describing to me a special mould that one of his rifles had been issued with that cast the bullets to fit firmly into the rifled bore.

I noticed a large bore muzzle loader on his site the other day that he'd had a similar arrangememt made for which cast bullets slightly oversize, then swaged them right to size -- with grooves to perfectly index the rifling -- it's said to load quickly and shoot very well.

I'm interested to hear what the bore/groove measurements are on your gun, if they are the same from tube to tube and rifle to rifle. I'm also interested to hear what the rifling arrangement is in those tubes.
Is this a 'composed pair' or an actual *made as a pair* pair of percussion double rifles.

Do you have the case for them?
Is there a vacancy in that case that would suggest that it came with a conical bullet mould and associated loading tools?

Show us some photos of this pair, we'll get to the bottom of this interesting mystery.
If what you have there is indeed a proper pair of guns, getting them to both throw all the shots into the same group will be a great accomplishment.

I will stay tuned here to see what you have to say...

--Tinker