First of all, you need the ability
Jeweler
Turns two identical accessories into a higher grade accessory. It also increases the accessory drop rates.
Apprentice 2 Identical Accessories >> 1 Higher Grade Accessory, Accessory Drop Rate + 25%
Journeyman 2 Identical Accessories >> 1 Higher Grade Accessory, Accessory Drop Rate + 50%
Master 2 Identical Accessories >> 1 Higher Grade Accessory, Accessory Drop Rate + 100%

if have the same HP + HP ring you can combine it.