Two enormous cauldrons do not work together. Yet an hour ago my guy with an enormous cauldron found a wineskin in battle--and they do work together, he's making 150 supply.
Yup, duplicates of the same item won't work, although there's what appears to be a bug with this for commanders capable of carrying more than 2 misc items.
Otherwise you'd never need to forge a treelord staff (2 thistle maces would be just as good) for example