So I have a high str charm and a high dex charm, but the stats for either only show when said charm is in the primary equip slot, while the stats in the second equip slot don't show. Does this mean that I have hidden stats drawn out of the secondary charm slot, or does it mean that I wasted coin buying two nice charms when I only needed one?

The secondary charm slot, also referred to as accessory charm ( acc. charm in short ) wont give you any stat points except enhanced effect.
This directly effects the charm itself and not your main charm or weapon for instance.

The efficiency is reduced by 25%, but in catacombs also mana costs are reduced by 25%.
Usually its the position where you put a heal charm in and have the stat charm in the main slot.