How to determine the source (computer) of an event in DropBox

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.

They can still share folders and files with you through the web interface, and vice versa - you can share folders and files with them, which they can view through the aforementioned webapp. Only difference is they won't have those files stored locally (unless they choose to manually download them).