TwoApp works without the use of a third-party server. There is no data being sent to me or any other third parties. TwoApp establishes a connection to Whatsapp's webservers and talks to no one but those very servers.
As no data is being stored externally and all data stays on your iPhone and your Watch, simply uninstalling the app will remove the reference between the app and its data.
Basically, no one but you and your chat partner(s) you send those messages to. For more info on how Whatsapps' encryption works, please refer to their documentation.