I need to customize User Registration. I inherited my control from UserModuleBase. I checked out definitions for AddUser (UserId=Null.NullInteger) & IsUser (uses Request.Authenticated in its definition).

As both do not use the same technique (UserId or Authenticated) to arrive at their conclusion, This is just to confirm that these 2 properties can never be True simultaneously.

Is there any scenario, where these 2 would be true simultaneously???

I asked wether they (IsUser & AddUser) can be true simultaneously?

IsUser checks whether a Member is editing his own account.

In your example, when an Admin is adding a Member, he is not editing hos own account, so IsUser would be False.

And when a User is self-registering, he is not authenticated, so IsUSer would be obviously false!!!