Affiliate commission on subsequent sales

I'm setting up an affiliate system for my store and everything is working so far. However, I had a request from one affiliate. He wants his sales to generate the following commissions:

1st sale for a customer: 15% commission

Subsequent sales for the same customer: 10% commission

Right now I have the affiliate plan set up to pay 15% commissions on every sale. Any ideas on how to set up the above?