To fulfill the requirement that the data in the sandbox should never be mixed with data in production during testing cycles, Northern Trail Outfitters should implement two separate Marketing Cloud instances. A Marketing Cloud instance is an independent environment that contains its own data, configuration, and functionality. Having two separate instances for sandbox and production will ensure data isolation and security during testing cycles.References:https://help.salesforce.com/s/articleView?id=sf.mc_overview_marketing_cloud_instances.htm