They were both in word 2007(open xml) format.
I searched online everywhere for a way to merge them.
All the links were about using the altChunk element. I have a problem with that. The entire document gets put in there and that is a lot of unneeded data(like header and footer etc.).
So I did it another way:
1. cloned the second document's body
2. cloned all it's children and added them to the first document's main part