I'm not sure I'm following, but there is one aspect which I should explain - PhotoPost uses the U_UserName field for login where Threads uses the U_LoginName. You can change line 27 in the forums/threads.php to use the other field if you want; however, when I tried to change to this back in version 3.X, I got flack from our Threads users who wanted it back the way it was (which did seem a little confusing to me, but thats what they wanted).
If you want both systems to key off the same U_LoginName, you only need change that one line.