Why are there two separate orders for one-time and prepaid products?

This behavior is expected. When a one-time product is processed with a prepaid shipment, it creates a separate order because it requires its own Charge ID. Prepaid shipments reuse the Charge ID from the initial prepaid charge, and a one-time product cannot share that same ID.

However, if the one-time product is processed with the initial prepaid charge, the system combines them into a single order.