This card can be used as a substitute for red eyes b. dragon, but not for a generic warrior monster. The thing you're doing wrong is trying to fuse 2 re-eyes b. dragon together. You need a warrior on the field to make it work.
So, it cannot be used to substitute a generic monster. For example, I tried to fusion summon Dark Cavalry that requires Dark Magician and Warrior monster, when I have Dark Magician and Hex-sealed on hand, I cannot do it.
This isn’t right, I’ve tried several times with an assault dragon In hand hex on field and can’t summon with polymerization amulet dragon witch would be using hex as sub for dark magician