Do you need both air filters on Subaru Forester?

My guess is that you're talking about both of the air intake filters, not the cabin air filter. In response to that, no, you don't need both air filters. I typically only use the first one, not the one very close to the engine intake because my car rarely uses the snorkel. Using both can actually reduce your gas mileage because it obstructs the airflow more. Mine's a '99 with almost 150k on it, and it's still running just fine with one. Hope that helps.