how to cause fusion which will then cause a summon

so i'm trying to make a mod which adds a new stage in the game. i've added the new armour set, tool set and a new boss aslong as a new gem but the way i plan to have the boss summoned is by dropping two items onto each other and so i assume that i use an event but i can't find anything that talks about an item being dropped nearby the current item summoning an entity. am i just not smart enough to understand it or is it actually not possible?