WHM username question

I have 2 domain names with 2 usernames setup on WHM:

domain1.com - domain1
domain2.com - domain2

I would like domain2.com website visitors to upload photos to a domain1 path. Unfortunately I can't, I think the problem is with the usernames and the fact that the path where I would like to add the photos belongs to domain1 user. The chmod is 777.
How can I set this up on WHM so domain2.com visitors can upload photos to a domain1 username path?

Thanks.

 

How to do this if both domain names are already setup?
They do need to have different files, just one directory where users post photos should be the same and both scripts (actually 2 different scripts from the 2 different domains) should be able upload to this directory.

 

No more suggestions?