Can multiple types of armor be stacked?

A character in my ongoing Exalted 2e game has an artifact chain shirt. Artifact chain shirts are described as being able to be worn under ordinary clothing (it has no mobility penalty or fatigue value). He’s also recently acquired a (non-artifact) reinforced breastplate. He’d like to wear them both. I don’t see any rules describing whether this can be done, what net protective effect it would have, or what effect it would have on mobility and fatigue.

Can multiple types of armor be stacked? How would I adjudicate this?

The character in question is pretty fragile so I’m not entirely opposed to his getting some kind of benefit, but I worry about setting a precedent that will bite me later.