Will there be an "&&" in Token string?

1) We don't have a documented guarantee of the exact format of Dropbox OAuth 2 access tokens, but the current implementation does not use "&". 

2) You can create and use multiple access tokens for the same app-user pair. Clicking "Generate" a second time will not invalidate the first access token.

3) An access token is all that is required to access an account, to the extent allowed by the corresponding app's permission level. You do not need to provide any additional information or certificate. Note that all Dropbox API calls do require TLS (i.e., as you can see from the https:// in your call, for instance), but that is all generally handled automatically for you.

Dropbox API OAuth 2 access tokens currently don't expire automatically, though they can be revoked.

Users can manually revoke tokens themselves, e.g. via:

https://www.dropbox.com/account/connected_apps

Apps can also revoke access tokens via:

https://www.dropbox.com/developers/documentation/http/documentation#auth-token-revoke

This being the case, you only need to process the authorization once, and save the access token for all future calls for that user. If an existing access token is invalid, (in which cases calls will return 401) you should prompt the user to re-authorize your app.