Book 2 session but only pay once

Hi, I offer an initial session that comprises of 2 appointments. Payment for both appointments is made when the client books the first appointment. How do I set it up that the client can book the second appointment without payment? At the moment in Booking Page Options I have selected Links On Confirmation Page for Schedule Second Session...unfortunately when the client clicks the link it looks for payment again. Is there a way for the link to the second appointment to have no payment required. I hope this makes sense.