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!

It is my theory that these kids are intelligent and this is the normal way they communicate with each other, kind of like solving puzzles.

"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 from their point of view 1 guy is the Father and 1 guy is the Elder brother (from their mother's point of view 1 guy is the Father of her children and 1 guys is her son). "One plus one, does it make one?" means "both are the same", not like a question or saying that is something which gives them so much weight to take and accept. 1+1=1!