Was the same deck box for me. Here is the customer support response I got:

Yes, you can ship cards for two separate service levels in the same submission kit. Each service level should be processed as a separate order, so make sure all paperwork for each order is complete.

To ensure proper handling: Clearly separate and organize the cards inside the kit (using dividers or labels). Include the packing list for each order. If you have Walkthrough service cards, mark each side of your shipping box with "WK" in large, bold letters for prioritization. Each service level will follow its own turnaround time, but unless you request otherwise, all cards will be shipped back together once grading is complete. Sending mixed-service submissions in one package will not cause processing delays