This is my theory that these kids are intelligent and the normal way they communicate with each other like solving puzzles, but anyway  **"One plus one, does it make one?"** means 

they are looking for 2 guys one is their father and one is their mother's lost child i.e their elder brother. So in their perspective **1 guy is Father** and **1 guy is Elder brother** (in their mother's perspective 1 guy is Father of her children and 1 guys is her son).  "One plus one, does it make one?" means **"Both are same"** not like a question or saying that is something which gives them so much weight to take and accept. **1+1=1!**