If a user tries to log into Site #2 (assuming already registered on Site #1) they will be advised the user account does not exist.
If the user goes to Site #2 (assuming already registered on Site #1) and tries to request a password reset, they will be advised the user account does not exist.
If the user goes to Site #2 (assuming already registered on Site #1) and tries to register with exactly the same Username and different Password as they used on Site #1, DNN will display an error.
If the user goes to Site #2 (assuming already registered on Site #1) and tries to register with exactly the same Username and Password as they used on Site #1, DNN will link the two accounts.
If the user goes to Site #2 (assuming already registered on Site #1) and tries to register with a different Username and Password to what was used on Site #1, DNN will create a separate account.