Conversion question for calibre

Hi. I don't know if anyone has had this issue before, but advice would be great. I've been trying to convert two books with different names but in the same series. Calibre has done the coversion with no issue, however when I open the books on my reader software the books seem to replace each other.
like books 1 and 3 are interchangeable for some reason. I cannot seem to get both into the reader at the same time. - the reader software I am using is Sony reader for PC.

thanks:help: