How to specify different FTP user in deployment credentials for each Azure Website?
No!
The deployment credentials are bound to your Microsoft Account (Live ID) and are same for all the web sites that your account has access to. These are called user level credentials.
There is also site level credentials - this are auto generated and auto managed by the KUDU. You cannot change it to a value of your own. Usually it is a long long string for username and same for password.
You can learn more about Web Site Credentials at the WIKI page for the KUDU project. What is KUDU? KUDU is the project that runs Azure Web Sites.
My understanding about your concern is that you want to give out FTP access to a 3rd party, and you don't want them to have access to the rest of the Web Sites you have. Well, currently this is partially possible - only with the Site-Level credentials, over which you have no control (username / password).