Two of the same phones - backup help needed

We have two phones, same type so how do we change the backup destination on the cloud. Right now all pictures from both phones end up dropping pictures into the same directory! A mess to straighten out.
How do you change what directory to back up to? Both destinations Samsung SM-G998U Camera Backup end up the same!

I have NO idea.

But the phones have an associated user account, right? Which has dedicated shares? Try having each phone with a separate user account on the NAS.