a simple workaround is: add the folder TWICE, so they appear like 2 folders, but with same content.
i'll give you instructions, but i didn't test so be careful
1. add the folder twice (when asked, click "real" folder)
2. give them 2 different names, like "upload" and "download"
3. on one folder you will hide all files. This may be achieved in several ways, maybe the easier is to use a "file filter" with this value: \*
4. the other folder shall be protected, and give access only to "a"
You can vary this approach in many ways. Example: you may put the "download" folder INSIDE the "upload", and also you could enable "list protected items only for allowed users".