I think those would be artifacts. Artifacts have different rules and are found in the DMG.
In most mythology and folktales both cloaks and pendants are used for protection thus the absence of the benefit from both. Also, you would not know what there worn items do because they could be...
Since both items do the same thing, unless they change this from 3E, you can not benefit from both anyway since they apply it in the same way. Thus, it does not matter what the reasoning is, it is pointless to use both.
From D&D XP Seminar: Critical Hits
Communicate key concepts for running a great game. Players will NOT need to own this.
How to build encounters, adventures, campaigns
Rules for customizing monsters and creating NPCs
More than 20 templates, from the Lich to Scion of the Flame
Traps, artifacts...