I'm trying to set up a system where when a member joins, they can add another member at a reduced rate. I want the second member to log in and sign up, checking a box for a release for. I thought the easiest way to do this would to give a discount code of 100% for the second member and the first member would pay for both. I know I could create a new discount code for each member but I'd rather not fill up the database with one time discounts, so I would like to hook into the code that the Apply button uses to examine the code entered and then change the amounts if it passes muster. I thought I could just use hook_civicrm_preProcess and change the amount on the form but this doesn't work. Is there a simple way to do this?