When sharing a Dropbox folder, whose quota is “billed”

You could try this method posted on Mac OS X Hints:

Just stick the folder of files into your public folder, and head to the Dropbox webapp. Click on the folder you want to share, then click the “Get Link” button at the top of the page. Add ?dl=1 to the end of it, and send that link to your friends. It’ll zip the file up for them and download it to their computer. (Lifehacker synopsis)

Try This:

Cust A shares folder with You. You share Folder with Cust B. (Folder stays same, no copy.)
No Auto-sync will be there, but maybe they both will see each other.

The foolproof plan can be:

Cust A shares with You (Copy A). You Make Copy B and share it with Cust B. (Now you have two copies of folder.)
Possibly no auto-sync, but for sure Cust A & B cannot see each other. You could use something like Microsoft Auto-Sync Toy (with Echo action) to sync.