How to know which user is owner of a folder in Dropbox

I'm afraid it comes out of both of your quotas: https://www.dropbox.com/help/59

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.