How can I be sure that both catalyic converters need replacing?

The problem you are describing is very common in a Mercedes-Benz. The catalytic converter core come loose causing a rattling noise and will eventually set a code for improper catalyst function triggering a check engine light. I recommend calling your local Mercedes-Benz dealership to see if it is covered under the emissions warranty for the vehicle. In addition, since it is a very common problem with Mercedes-Benz vehicles, cost assistance may be available if you are out of the warranty window.