No mater how you look at it it took 2 drops to make the big drop.
So I would look at it like this 1 drop + 1 drop= 2 drops to make 1 big drop if you put them on top of each other.

Its not the end result of the drops it's how many drops it took to reach the size of the bigger drop in this case it took 1+1=2 .
Same as it takes 1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+ 1+1+1+1+1+1+1+1+1+1+1+1+1+1+1=40 rain drops to make 1 puddle.