If doing this manually and with Windows (not sure about MAC) Place the contents of the first zip into the folder. When you move the contents of the second zip to the folder windows will say that duplicate files exist and ask what you want to do. If they are the same name size and date they will be replaced. If they differ then you can elect to choice which file to keep, or keep both and Windows will add a number to the second to make the files different.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13