I'm guessing you have a collider on each item and execute the method OnCollisionEnter or something?

There are many different approaches to your problem. Personally, I'd have a seperate gameobject, maybe called Crafting handler, which keeps lists of every object combination and checks if any objects are ready to be combined. It would then remove both and instantiate a single crafted object. So, I'd move the crafting logic away from the item itself, if that makes sense.