Sequence Diagram and executing another use case

I would like to know how I can represent passing a process to another use case. Let us say that I have two sequence diagrams. Diagram 1) Sequence diagram that lets a customer select itineries and asks the customer to confirm the itinery. Diagram 2) Login sequence diagram where the user enters user info and logs into the system.

For the customer to confirm an itinery, he/she has to be logged-in. When the customer selects to confirm itinery in diagram 1, and when he/she is not logged in, I want to pass the control to Log-in sequence diagram. How do I represent this in diagram 1. Should I have the Diagram 2 as an actor in diagram 1. Or I should not be doing any passing to different diagram but repeat the same process of diagram 2 in diagram 1.

Any suggestions would be highly appreciated.

You might find http://www.agilemodeling.com/style/sequenceDiagram.htm#Figure3 to be of interest.

- Scott