Multiple AHU from shared OA

Wayne, Your situation isn't entirely uncommon on older existing systems. We've run into similar cases with mixing chambers associate with large existing units. Unfortunately, you do need to be able to measure each SCU's outside air intake individually. If you have a common return and OA mixing chamber that both units share you're unlikely to achieve this credit without a system retrofit. You could submit an "alternative compliance" narrative stating you are meeting the intent of the credit but have an existing system that only allows you to measure the sum of both SCU's OA quantities. I wouldn't hold out on this credit, not all projects are suitable.