Share a folder with Dropbox users but prevent them from modifying

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.

The best option is to roll-your-own. There is an opensource option that takes a bit of set-up, but is fairly good and does respect permissions, however, it does require you to generate everything for your clients. In any case - worth checking into:

ownCloud

ownCloud gives you universal access to your files through a web interface or WebDAV. It also provides a platform to easily view & sync your contacts, calendars and bookmarks across all your devices and enables basic editing right on the web. Installation has minimal server requirements, doesn’t need special permissions and is quick. ownCloud is extendable via a simple but powerful API for applications and plugins.

ownCloud started with a keynote by Frank Karlitschek at Camp KDE’10 where he talked about the need of a self-controlled free and open source cloud.