I need to pass multiple SKUs to a backend system for certain products. Any idea how to do this?

For example, if a customer orders a years supply of a product, the product is shipped monthly. In order for the backend system to do this it needs to get 2 SKUs for that product. So I need to send the backend system a SKU (product model) for the initial shipment with a price and them a second SKU for the reoccurring shipments (billing and fufillment happens externally from ZenCart)

This is not the way I would have set it up, butI have to accomodate for it.

Thanks