Imagine you have one use case “register contract” and one extension “attach document”. The two actors are not related at all (I mean, one of them do not extend the other one); both can register a contract but only the “client” can also attach documents.
How do you represent that? Is my example right or should I trace a line from the client to the “register contract” use case too?

In this case I would use two diagrams, one per actor