Problem with combining writable PDFs

I have two writable PDFs that I want to combine on Adobe Acrobat. They each have the same two pages but each has been filled out dirrerently. When I combine them, the first two pages on each is correct but the secod page on the second file is a duplicate of the second page on the first file. So basically I have one file with pages A and B and one with pages C and D but when I combine them I get A, B, C and B. I assume it has something to do with them being writiable PDFs, but I don't know why Acrobat would change them.