I can't for the life of me find anything about this, though I've seen several mods that incorporate them. I've just downloaded a mod that contains, among other items, a pair of necklaces. Both are supposed to be visible on my character when she wears them, but only one actually is. How would I go about making the other one visible? Is there something I can refer to?

In Xenn's marksman overhaul, some of the guards wear quivers. This is achieved by replacing it at the slot of right pauldron.

So my guess is the necklace is assigned a body part which it replaces when equipped. So looking at the necklace that appears on the player in the CS might help.
I am not an expert in these but just nudging in the right direction.

EDIT: I meant NX9s guard not Xenn's marksman mod

That was the first thing I checked. Both necklaces are clavicle body parts and assigned to replace the left pauldron.

Does it have mesh for the body part? thats my guess......
but then big yellow exclamation should have appeared when equipped...... I dont know........sorry, but somebody who has experience in this should help.