Allow a code to be made with a "book _, get %/$ off _" so that one service has to be booked in order for a discount to be applied to the second service. For example, "get 50% off enter service when you also book a enter service in the same appointment!". This would limit the need for manual tracking if an offer is applied to an appointment and one of the two services is cancelled so the client doesn't get the discount they don't qualify for. Also eliminated the need to manually apply the discount at checkout.