You can try using smigrate.exe to transfer contents  from one site  to another.

Smigrate.exe moves sites, not documents. The only way I know of to do this is to manually use the web folder client to move documents. good copy/paste.

How do I find the web folders ???

Here's the structure

Sites -> Subsite1 -> DocLib1

Sites -> Subsite2 -> DocLib2

Also, when the document was loaded in the Doc Library, it has attributes
i.e. user defined columns added to it. By using copy paste I am not sure
what will happen to this info. that would have been stored in the database
with this document. So not confusing you too much, does these attributes get
deleted (on their own), do they get moved to the new sub site's doc lib, or
do I need to ensure I manually delete these attributes some way.

It's setting up a network place to the two document libraries and using
port 80 to transfer documents  between the two libs.