For multi-dog households, you need one Fi Collar + one Fi GPS/LTE Subscription/Membership per pup to track each dog remotely.
Each Fi Collar operates as its own cellular device, so we need to establish one AT&T line per dog. This enables you to monitor each individual dog's whereabouts.
A subscription/membership gives you access to the full capabilities of your dog's Fi Collar - allowing Fi remote access to your dog's location and activity. The subscription/membership cost covers the GPS/LTE connection to your dog's Fi collar. See: What's Included in My Fi GPS/LTE Subscription? If you did not add a subscription at checkout for a Fi Series 2, you can add a subscription after you checked out. See: How Do I Add a Fi GPS/LTE Subscription?