"Somebody Stupefied a Death Eater on top of the Tower after Dumbledore died. There were also two broomsticks up there. The Ministry can add two and two, Harry."
Harry Potter and the Half-Blood Prince
I deduce that "add two and two" is a variant of put two and two together. If so, any reason why it's been "two and two", not "one and one", etc. "One and one" looks more simple.