For this to works, one of the similar equipment must be LOWER in level. Haven’t tested for thrown/wands and for items with same level. So if one of LOWER leveled equipment are CURSED and EQUIPPED (can’t be unequipped), for some reason it will not be destroyed by reforging process.
First case I encountered was with Displace Plate +2 (equipped) and Anti-Magic Plate +3. Reforging result was Anti-Magic Plate +4 but somehow the cursed Plate +2 were still intact and equipped. Then I tried again with Curse Infused RoW +0 (equipped) and RoW +9. I was able to get RoW +11 by reforging twice using the same cursed RoW + 0, and it was still intact after the second reforge! Proof for the second case below via imgur.
Just realized that the title could be misleading bcos it’s not really multiplied per se, it’s just that you only need 1 other similar equipment to do two reforging AND you get to keep the lower level one. Sorry, english are not my first language, cant think of better wording.